Title of Invention: Managing impression opportunities for a set of content items associated with goals to guarantee achievement of the goals within a specified period of time Abstract: An online system guarantees achievement of an impression goal and a reach goal associated with a set of content items received from a content-providing user of the online system within a period of time associated with the goals. To ensure that the goals are achieved within the period of time, the online system may adjust values of filters associated with the set of content items that control a number of impression opportunities for which the set of content items will be eligible for presentation to users of the online system. The online system may compute a normalized ratio throughout the period of time associated with the goals based on the goals and the progress made by the online system towards achieving the goals. Based on the normalized ratio, the online system may track the performance of the content items and adjust the values of the filters. Detailed Description: Part general_description_paragraphs: Part BACKGROUND: This disclosure relates generally to online systems, and more specifically to managing impression opportunities for a set of content items associated with goals to guarantee achievement of the goals within a specified period of time. An online system allows its users to connect and communicate with other online system users. Users create profiles in the online system that are tied to their identities and include information about the users, such as interests and demographic information. The users may be individuals or entities such as corporations or charities. Because of the popularity of online systems and the significant amount of user-specific information maintained in online systems, an online system provides an ideal forum for content-providing users to share content by creating content items (e.g., advertisements) for presentation to additional online system users. For example, content-providing users may share photos or videos they have uploaded by creating content items that include the photos or videos that are presented to additional users to whom they are connected in the online system. By allowing content-providing users to create content items for presentation to additional online system users, an online system also provides abundant opportunities to persuade online system users to take various actions and/or to increase awareness about products, services, opinions, or causes among online system users. For example, if a content-providing user of the online system who volunteers for a non-profit organization creates a content item encouraging additional online system users to volunteer for the non-profit organization, the online system may present the content item to these additional online system users. To encourage content-providing users of an online system to create content items for presentation to additional online system users, online systems may “pace” the presentation of the content items in order to increase their effectiveness. For example, suppose that an advertiser has created an advertisement that is to be presented to users of an online system over the course of a month and that the advertiser has allocated a budget for this purpose. To increase the likelihood that the advertisement is presented to the greatest number of potential customers, rather than presenting the advertisement to users as frequently as possible at the beginning of the first day of the month until the budget has been spent, the online system may present the advertisement to users at a more even pace over the course of each day during the month until the budget has been spent. Online systems also may encourage content-providing users of an online system to create content items for presentation to additional online system users by guaranteeing the content-providing users that various goals associated with content items received from the content-providing users will be achieved within a specified period of time. For example, an online system may guarantee an advertiser from whom a set of advertisements associated with an advertising campaign was received that the set of advertisements will achieve an impression goal of at least 700 impressions within a specified week (e.g., the online system guarantees it will present the set of advertisements at least 700 times within the specified week). In this example, the online system also may guarantee the advertiser that the set of advertisements will achieve a reach goal of at least 300 online system users within the specified week (e.g., the online system guarantees it will present the set of advertisements to at least 300 different online system users within the specified week). To achieve one or more goals associated with a set of content items within a specified period of time while pacing the presentation of the content items, an online system may adjust a frequency with which the content items are presented to users of the online system on a daily basis. In the above example, each day during the specified week, the online system may compare the impression goal of 700 impressions to the number of impressions achieved by the set of advertisements. Based on the comparison, the online system may adjust the frequency with which the advertisements are presented to users of the online system (e.g., by increasing the frequency if the current rate at which the impressions are being achieved indicates that the goal will not be achieved by the end of the week or by decreasing the frequency if the current rate indicates that over-achievement of the goal will occur at the end of the week). Similarly, in the above example, each day during the specified week, the online system also may compare the reach goal of 300 online system users to the number of different online system users who have been presented with one or more of the set of advertisements. Based on the comparison, the online system may adjust the frequency with which the advertisements are presented to users of the online system who have not been presented with any of the set of advertisements. However, the number of opportunities that an online system may identify to present content to users of the online system often fluctuates. Therefore, adjusting the frequency with which content items are presented to users in the manner described above may lead to under-achievement of goals associated with the content items, which may discourage content-providing users from creating more content items for presentation to additional online system users. For example, suppose that a set of content items is associated with a reach goal that an online system has guaranteed will be achieved within a specified month. In this example, if the reach goal has not yet been achieved by the last day of the month, even if the online system has increased the frequency with which the set of content items will be presented to users of the online system who have not been presented with the content items, if the number of impression opportunities for these users is smaller than normal, the online system may fail to achieve the reach goal by the end of the month. Similarly, adjusting the frequency with which content items are presented to users of an online system in the manner described above also may lead to over-achievement of a goal, which may result in wasted opportunities that the online system may have used to present content items that are more likely to be relevant to users of the online system. In the above example, if the number of impression opportunities for the users is larger than normal, over-achievement of the reach goal may occur at the end of the month. Moreover, over-achievement of a goal may be more likely to occur for content items that are associated with multiple goals that online systems have guaranteed will be achieved within the same period of time. Once a goal associated with a set of content items is achieved, an online system may adjust the frequency with which the content items are presented to users of the online system to focus on other goals associated with the content items that have not yet been achieved. For example, suppose that a set of content items is associated with an impression goal and a reach goal that the online system has guaranteed will be achieved within a week and prior to the end of the week, the impression goal has been achieved, but the reach goal has not yet been achieved. In this example, the online system may stop presenting the content items to users of the online system who already have been presented with any of the content items, but may increase the frequency with which the content items are presented to users who have not been presented with any of the content items. Continuing with this example, since the online system must still present the content items to users of the online system who have not been presented with any of the content items in order to achieve the reach goal, over-achievement of the impression goal is necessary in order to achieve the reach goal. Part SUMMARY: The foregoing description of the embodiments has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the patent rights to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. Some portions of this description describe the embodiments in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof. Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described. Embodiments also may relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. Embodiments also may relate to a product that is produced by a computing process described herein. Such a product may comprise information resulting from a computing process, where the information is stored on a non-transitory, tangible computer readable storage medium and may include any embodiment of a computer program product or other data combination described herein. Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the patent rights be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the patent rights, which is set forth in the following claims. Part DETAILED DESCRIPTION: System Architecture FIG. 1 is a block diagram of a system environment 100 for an online system 140. The system environment 100 shown by FIG. 1 comprises one or more client devices 110, a network 120, one or more third-party systems 130, and the online system 140. In alternative configurations, different and/or additional components may be included in the system environment 100. The client devices 110 are one or more computing devices capable of receiving user input as well as transmitting and/or receiving data via the network 120. In one embodiment, a client device 110 is a conventional computer system, such as a desktop or a laptop computer. Alternatively, a client device 110 may be a device having computer functionality, such as a personal digital assistant (PDA), a mobile telephone, a smartphone or another suitable device. A client device 110 is configured to communicate via the network 120. In one embodiment, a client device 110 executes an application allowing a user of the client device 110 to interact with the online system 140. For example, a client device 110 executes a browser application to enable interaction between the client device 110 and the online system 140 via the network 120. In another embodiment, a client device 110 interacts with the online system 140 through an application programming interface (API) running on a native operating system of the client device 110, such as IOS® or ANDROID™. The client devices 110 are configured to communicate via the network 120, which may comprise any combination of local area and/or wide area networks, using both wired and/or wireless communication systems. In one embodiment, the network 120 uses standard communications technologies and/or protocols. For example, the network 120 includes communication links using technologies such as Ethernet, 802.11, worldwide interoperability for microwave access (WiMAX), 3G, 4G, code division multiple access (CDMA), digital subscriber line (DSL), etc. Examples of networking protocols used for communicating via the network 120 include multiprotocol label switching (MPLS), transmission control protocol/Internet protocol (TCP/IP), hypertext transport protocol (HTTP), simple mail transfer protocol (SMTP), and file transfer protocol (FTP). Data exchanged over the network 120 may be represented using any suitable format, such as hypertext markup language (HTML) or extensible markup language (XML). In some embodiments, all or some of the communication links of the network 120 may be encrypted using any suitable technique or techniques. One or more third-party systems 130 may be coupled to the network 120 for communicating with the online system 140, which is further described below in conjunction with FIG. 2. In one embodiment, a third-party system 130 is an application provider communicating information describing applications for execution by a client device 110 or communicating data to client devices 110 for use by an application executing on the client device 110. In other embodiments, a third-party system 130 (e.g., a content publisher) provides content or other information for presentation via a client device 110. A third-party system 130 also may communicate information to the online system 140, such as advertisements, content, or information about an application provided by the third-party system 130. FIG. 2 is a block diagram of an architecture of the online system 140. The online system 140 shown in FIG. 2 includes a user profile store 205, a content store 210, an action logger 215, an action log 220, an edge store 225, a goal store 230, a filter store 235, an achievement module 240, an achievement store 245, a normalization module 250, a graphing module 255, a filter initialization module 260, a filter evaluation module 265, a filter adjustment module 270, a content selection module 275, a user interface module 280, and a web server 285. In other embodiments, the online system 140 may include additional, fewer, or different components for various applications. Conventional components such as network interfaces, security functions, load balancers, failover servers, management and network operations consoles, and the like are not shown so as to not obscure the details of the system architecture. Each user of the online system 140 is associated with a user profile, which is stored in the user profile store 205. A user profile includes declarative information about the user that was explicitly shared by the user and also may include profile information inferred by the online system 140. In one embodiment, a user profile includes multiple data fields, each describing one or more user attributes for the corresponding online system user. Examples of information stored in a user profile include biographic, demographic, and other types of descriptive information, such as work experience, educational history, gender, hobbies or preferences, locations and the like. A user profile also may store other information provided by the user, for example, images or videos. In certain embodiments, images of users may be tagged with information identifying the online system users displayed in an image. A user profile in the user profile store 205 also may maintain references to actions by the corresponding user performed on content items in the content store 210 and stored in the action log 220. The user profile store 205 also may store user-identifying information associated with users of the online system 140. In some embodiments, user-identifying information associated with a user of the online system 140 may include personally identifiable information. Examples of personally identifiable information that may be associated with a user of the online system 140 may include a full name, a home address, a phone number, an email address, a user identifier (e.g., a username, a user identification number, or a cookie identifier), a client device identifier (e.g., an IP address), a browser identifier (e.g., a user agent), etc. User-identifying information also may include information that potentially may be combined with other personal information to identify a user of the online system 140 (e.g., an age, a gender, a geographic region, etc.). While user profiles in the user profile store 205 are frequently associated with individuals, allowing individuals to interact with each other via the online system 140, user profiles also may be stored for entities such as businesses or organizations. This allows an entity to establish a presence in the online system 140 for connecting and exchanging content with other online system users. The entity may post information about itself, about its products or provide other information to users of the online system 140 using a brand page associated with the entity's user profile. Other users of the online system 140 may connect to the brand page to receive information posted to the brand page or to receive information from the brand page. A user profile associated with the brand page may include information about the entity itself, providing users with background or informational data about the entity. The content store 210 stores objects that each represent various types of content. Examples of content represented by an object include a page post, a status update, a photograph, a video, a link, a shared content item, a gaming application achievement, a check-in event at a local business, a page (e.g., brand page), an advertisement, or any other type of content. Online system users may create objects stored by the content store 210, such as status updates, photos tagged by users to be associated with other objects in the online system 140, events, groups or applications. In some embodiments, objects are received from third-party applications or third-party applications separate from the online system 140. In one embodiment, objects in the content store 210 represent single pieces of content, or content “items.” Hence, online system users are encouraged to communicate with each other by posting text and content items of various types of media to the online system 140 through various communication channels. This increases the amount of interaction of users with each other and increases the frequency with which users interact within the online system 140. The content store 210 may store (e.g., as shown in step 302 of FIG. 3) objects representing content items received from content-providing users of the online system 140. Each content item received from a content-providing user may be associated with various types of information that may be used to uniquely identify the content item. For example, a content item may be identified based on information identifying a content-providing user of the online system 140 from whom the content item was received and on a title or other unique identifier assigned to the content item by the content-providing user. As an additional example, a content item may be identified based on a unique identification number assigned by the online system 140 to each content item maintained in the online system 140. In the above examples, the information that may be used to uniquely identify each content item may be stored in association with an object representing the content item in the content store 210. Each content item received from a content-providing user also may be associated with one or more goals and a period of time in which the online system 140 may guarantee that each goal will be achieved. For example, if a set of content items received from a content-providing user of the online system 140 is associated with a goal, the online system 140 may guarantee that the goal will be achieved within a specified period of time (e.g., within a specified week or within a specified month). In this example, information describing the goal associated with each content item included among the set of content items received from the content-providing user and the period of time in which the goal is to be achieved may be stored in association with an object representing the content item in the content store 210. The action logger 215 receives communications about user actions internal to and/or external to the online system 140, populating the action log 220 with information about user actions. Examples of actions include adding a connection to another user, sending a message to another user, uploading an image, reading a message from another user, viewing content associated with another user, and attending an event posted by another user. In addition, a number of actions may involve an object and one or more particular users, so these actions are associated with those users as well and stored in the action log 220. The action log 220 may be used by the online system 140 to track user actions in the online system 140, as well as actions in the third-party system 130 that communicate information to the online system 140. Users may interact with various objects in the online system 140, and information describing these interactions is stored in the action log 220. Examples of interactions with objects include: commenting on posts, sharing links, checking-in to physical locations via a mobile device, accessing content items, and any other suitable interactions. Additional examples of interactions with objects in the online system 140 that are included in the action log 220 include: commenting on a photo album, communicating with a user, establishing a connection with an object, joining an event, joining a group, creating an event, authorizing an application, using an application, expressing a preference for an object (“liking” the object), and engaging in a transaction. Additionally, the action log 220 may record a user's interactions with advertisements in the online system 140 as well as with other applications operating in the online system 140. In some embodiments, data from the action log 220 is used to infer interests or preferences of a user, augmenting the interests included in the user's user profile and allowing a more complete understanding of user preferences. The action log 220 also may store user actions taken on a third-party system 130, such as an external website, and communicated to the online system 140. For example, an e-commerce website may recognize a user of an online system 140 through a social plug-in enabling the e-commerce website to identify the user of the online system 140. Because users of the online system 140 are uniquely identifiable, e-commerce web sites, such as in the preceding example, may communicate information about a user's actions outside of the online system 140 to the online system 140 for association with the user. Hence, the action log 220 may record information about actions users perform on a third-party system 130, including webpage viewing histories, advertisements that were engaged, purchases made, and other patterns from shopping and buying. Additionally, actions a user performs via an application associated with a third-party system 130 and executing on a client device 110 may be communicated to the action logger 215 for storing in the action log 220 by the application for recordation and association with the user by the online system 140. In embodiments in which content items presented to users of the online system 140 are associated with various goals, the action log 220 may store (e.g., as shown in step 320 of FIG. 3) information describing the progress made by the online system 140 towards achieving one or more goals associated with a set of content items. For example, upon each presentation of a content item associated with an impression goal to a user of the online system 140, information received by the online system 140 describing the presentation may be stored in the action log 220 (e.g., by the action logger 215). As an additional example, if a content item is associated with a conversion goal, information describing each conversion associated with the content item received by the online system 140 may be stored in the action log 220 by the action logger 215. Examples of information describing the progress made by the online system 140 towards achieving one or more goals associated with a content item may include user-identifying information for a viewing user of the online system 140 to whom the content item was presented or who performed an action associated with the content item (e.g., an IP address associated with the user, a username or a user ID associated with the user, etc.), information identifying the content item, information describing one or more goals associated with the content item, and information indicating a date and a time at which the content item was presented or a date and a time at which the user performed the action associated with the content item (e.g., in a timestamp). For example, upon presenting an advertisement associated with an impression goal to a user of the online system 140, the action logger 215 may receive an IP address associated with the viewing user, information identifying an advertising campaign associated with the advertisement, information indicating that the advertisement is associated with the impression goal, and a date and a time at which the advertisement was presented. In this example, the action logger 215 may store this information in the action log 220. In one embodiment, the edge store 225 stores information describing connections between users and other objects in the online system 140 as edges. Some edges may be defined by users, allowing users to specify their relationships with other users. For example, users may generate edges with other users that parallel the users' real-life relationships, such as friends, co-workers, partners, and so forth. Other edges are generated when users interact with objects in the online system 140, such as expressing interest in a page in the online system 140, sharing a link with other users of the online system 140, and commenting on posts made by other users of the online system 140. In one embodiment, an edge may include various features each representing characteristics of interactions between users, interactions between users and objects, or interactions between objects. For example, features included in an edge describe rate of interaction between two users, how recently two users have interacted with each other, the rate or amount of information retrieved by one user about an object, or the number and types of comments posted by a user about an object. The features also may represent information describing a particular object or user. For example, a feature may represent the level of interest that a user has in a particular topic, the rate at which the user logs into the online system 140, or information describing demographic information about a user. Each feature may be associated with a source object or user, a target object or user, and a feature value. A feature may be specified as an expression based on values describing the source object or user, the target object or user, or interactions between the source object or user and target object or user; hence, an edge may be represented as one or more feature expressions. The edge store 225 also stores information about edges, such as affinity scores for objects, interests, and other users. Affinity scores, or “affinities,” may be computed by the online system 140 over time to approximate a user's interest in an object or in another user in the online system 140 based on the actions performed by the user. A user's affinity may be computed by the online system 140 over time to approximate a user's interest in an object, a topic, or another user in the online system 140 based on actions performed by the user. Computation of affinity is further described in U.S. patent application Ser. No. 12/978,265, filed on Dec. 23, 2010 (U.S. Publication No. US 2012/0166532 A1, published on Jun. 28, 2012), U.S. patent application Ser. No. 13/690,254 (U.S. Pat. No. 9,070,141, issued on Jun. 30, 2015), filed on Nov. 30, 2012, U.S. patent application Ser. No. 13/689,969, filed on Nov. 30, 2012 (U.S. Pat. No. 9,317,812, issued on Apr. 19, 2016), and U.S. patent application Ser. No. 13/690,088, filed on Nov. 30, 2012 (U.S. Publication No. US 2014/0156360 A1, published on Jun. 5, 2014), each of which is hereby incorporated by reference in its entirety. Multiple interactions between a user and a specific object may be stored as a single edge in the edge store 225, in one embodiment. Alternatively, each interaction between a user and a specific object is stored as a separate edge. In some embodiments, connections between users may be stored in the user profile store 205, or the user profile store 205 may access the edge store 225 to determine connections between users. The goal store 230 stores (e.g., as shown in step 304 of FIG. 3) information describing goals associated with various sets of content items and a period of time associated with each goal. A goal may be associated with a set of content items received from a content-providing user of the online system 140 and a period of time in which the online system 140 may guarantee that the goal will be achieved. For example, a goal may be associated with a set of advertisements associated with an advertising campaign received from an advertiser or any other suitable content-providing user of the online system 140. In this example, the advertiser may specify a goal of 1,000 impressions of the set of advertisements and the online system 140 may guarantee that the goal will be achieved within a specified week. In embodiments in which a goal is associated with multiple content items, information describing the goal may be stored in the goal store 230 in association with information describing a period of time associated with the goal and information identifying each content item associated with the goal. In the above example, the online system 140 may store information describing the goal in the goal store 230 in association with information identifying the specified week and information identifying each content item associated with the goal. A set of content items may be associated with various types of goals. In some embodiments, a set of content items may be associated with an impression goal. For example, to achieve an impression goal of 500 impressions, one or more content items included among a set of content items associated with the impression goal must be presented to one or more users of the online system 140 at least 500 times. In various embodiments, a set of content items may be associated with a reach goal. For example, to achieve a reach goal of 300 online system users, one or more content items included among a set of content items associated with the reach goal must be presented to at least 300 different users of the online system 140. A set of content items also may be associated with a conversion goal, a click goal, or any other suitable type of goal that may be achieved by the set of content items. For example, to achieve a conversion goal of 200 purchases, 200 purchases must be made by online system users in association with presentations of one or more content items included among a set of content items associated with the conversion goal. As an additional example, to achieve a click goal of 700 clicks, online system users must click on one or more content items included among a set of content items associated with the click goal at least 700 times. As yet another example, to achieve a “like” goal of 150 likes, 150 online system users must express a preference for one or more content items included among a set of content items associated with the like goal. In some embodiments, a set of content items may be associated with multiple goals. In such embodiments, information describing each goal associated with the set of content items may be stored in the goal store 230 in association with information describing a period of time associated with the goal and information identifying each content item included among the set of content items. For example, suppose that a set of content items is associated with an impression goal and a reach goal. In this example, the online system 140 may store information describing the impression goal in the goal store 230 in association with information describing a period of time in which the impression goal is to be achieved and information identifying each content item included among the set of content items. Similarly, in this example, the online system 140 also may store information describing the reach goal in the goal store 230 in association with information describing a period of time in which the reach goal is to be achieved and information identifying each content item included among the set of content items. The goal store 230 is further described below in conjunction with FIG. 3. The filter store 235 stores (e.g., as shown in step 310 of FIG. 3) values of filters associated with various sets of content items maintained in the online system 140 that are associated with one or more goals. For each goal associated with a set of content items, a value of a corresponding filter may be stored in the filter store 235. For example, if a set of content items is associated with an impression goal, each content item included among the set of content items may be associated with an impression filter. In this example, a value of the impression filter may be stored in the filter store 235. In the above example, if the set of content items also is associated with a reach goal, each content item included among the set of content items also may be associated with a reach filter. In this example, a value of the reach filter also may be stored in the filter store 235. The value of each filter stored in the filter store 235 may control a number of impression opportunities for which one or more content items included among a set of content items associated with the filter will be eligible for presentation to users of the online system 140. In some embodiments, values of filters may correspond to probabilities. For example, suppose that the value of an impression filter associated with a set of content items corresponds to a probability of 10% (i.e., 0.1). In this example, upon identifying an opportunity to present content to a viewing user of the online system 140, there is a 10% probability that the content selection module 275 (described below) will identify one or more content items included among the set of content items as being eligible for presentation to the viewing user. The value of a filter associated with a set of content items may be stored in association with information identifying the content items, information describing a corresponding goal associated with the content items, and information describing a period of time associated with the goal. For example, a value of an impression filter associated with a set of content items may be stored in the filter store 235 in association with information identifying each content item included among the set of content items, information describing a number of impressions that must be achieved by the set of content items based on an impression goal associated with the set of content items, and a period of time in which the online system 140 has guaranteed that the impression goal will be achieved. The filter store 235 is further described below in conjunction with FIG. 3. The achievement module 240 identifies (e.g., as shown in step 322 of FIG. 3) information received by the online system 140 (e.g., by the action logger 215) describing progress made by the online system 140 towards achieving each goal associated with a set of content items within a period of time associated with the goal. For example, suppose that a set of content items is associated with an impression goal and that the action logger 215 receives information describing an impression achieved by a content item included among the set of content items for a viewing user of the online system 140. In this example, based on information identifying the content item included among the information received by the action logger 215, the achievement module 240 may access the goal store 230 or the content store 210 and retrieve information describing the impression goal and a period of time in which the impression goal is to be achieved. Continuing with this example, if the information describing the impression achieved by the content item was received by the action logger 215 at a time within the period of time in which the impression goal is to be achieved, the achievement module 240 may identify the information received by the action logger 215 as information describing progress made by the online system 140 towards achieving the impression goal. In the above example, if the set of content items also is associated with a reach goal, the achievement module 240 also may retrieve information describing the reach goal and a period of time in which the reach goal is to be achieved from the goal store 230 or the content store 210. Continuing with this example, if the information describing the impression achieved by the content item was received by the action logger 215 at a time within the period of time in which the reach goal is to be achieved, the achievement module 240 also may access the action log 220 and determine whether any content items included among the set of content items previously were presented to the viewing user within the period of time (e.g., based on user-identifying information associated with the viewing user included among the information received by the action logger 215). In this example, if none of the set of content items previously were presented to the viewing user within the period of time in which the reach goal is to be achieved, the achievement module 240 also may identify the information received by the action logger 215 as information describing progress made by the online system 140 towards achieving the reach goal. The achievement module 240 also may update (e.g., as shown in step 324 of FIG. 3) information describing progress made by the online system 140 towards achieving each goal associated with a set of content items within a period of time associated with the goal. The achievement module 240 may update information stored in the online system 140 (e.g., in the achievement store 245, described below), that describes progress made by the online system 140 towards achieving each goal associated with a set of content items during a period of time associated with the goal. In some embodiments, the achievement module 240 may do so by updating a count associated with each goal. In such embodiments, the achievement module 240 may increment a value of a count associated with a goal upon identifying information indicating that progress made by the online system 140 towards achieving the goal has been made within a period of time associated with the goal. For example, suppose that a set of content items is associated with an impression goal that the online system 140 has guaranteed will be achieved within a specified month. In this example, each time the achievement module 240 identifies information received by the action logger 215 describing progress made by the online system 140 towards achieving the impression goal during the specified month, the achievement module 240 may increment an impression count associated with the impression goal that is stored in the achievement store 245. In the above example, suppose that the content item also is associated with a reach goal that the online system 140 has also guaranteed will be achieved within the specified month. In this example, upon incrementing the impression count associated with the impression goal, if the achievement module 240 also identifies the information received by the action logger 215 as information describing progress made by the online system 140 towards achieving the reach goal, the achievement module 240 also may increment a reach count associated with the reach goal that is stored in the achievement store 245. The functionality of the achievement module 240 is further described below in conjunction with FIG. 3. As described above in conjunction with the achievement module 240, in some embodiments, information stored in the achievement store 245 may include a count associated with each goal associated with a set of content items that describes progress made by the online system 140 towards achieving the goal during a period of time associated with the goal. For example, the achievement store 245 may store an impression count associated with an impression goal associated with a set of content items, in which a value of the impression count may indicate a number of impressions achieved by the set of content items during a period of time in which the impression goal is to be achieved. As an additional example, the achievement store 245 may store a reach count associated with a reach goal associated with a set of content items, in which a value of the reach count may indicate a number of unique online system users for whom impressions of the content items have been achieved during a period of time in which the reach goal is to be achieved. As yet another example, the achievement store 245 may store a conversion count associated with a conversion goal associated with a set of content items, in which a value of the conversion count may indicate a number of purchases that have been made by online system users in association with presentations of the content items during a period of time in which the conversion goal is to be achieved. The achievement store 245 is further described below in conjunction with FIG. 3. The normalization module 250 computes (e.g., as shown in step 326 of FIG. 3) one or more normalized ratios used to track the performance of a set of content items associated with various goals. The normalization module 250 may compute the normalized ratios at periodic time intervals during a period of time in which the goals associated with the set of content items are to be achieved. For example, if a set of content items is associated with an impression goal and a reach goal that are to be achieved within a specified week, the normalization module 250 may compute a normalized ratio once every hour within the specified week. The normalization module 250 may compute a normalized ratio based on goals associated with a set of content items and on the progress made by the online system 140 towards achieving the goals. In some embodiments, the normalization module 250 may compute a normalized ratio as a ratio of an achievement ratio to a goal ratio. For example, if an achievement ratio has a value of 0.8 and a goal ratio has a value of 0.2, the normalization module 250 may compute a normalized ratio to have a value of 4 (i.e., 0.8/0.2). The normalization module 250 may compute a goal ratio to be a ratio of one goal associated with a set of content items to another goal associated with the set of content items. For example, the normalization module 250 may access the goal store 230 to retrieve information describing a reach goal and an impression goal associated with a set of content items. In this example, if the reach goal associated with the set of content items is to present the content items to at least 500 different online system users and the impression goal associated with the set of content items is to achieve at least 1,000 impressions of the content items, the normalization module 250 may compute the goal ratio to have a value of 0.5 (i.e., 500/1,000). Similarly, the normalization module 250 may compute an achievement ratio to be a ratio of the progress made by the online system 140 towards achieving one goal associated with a set of content items to progress made by the online system 140 towards achieving another goal associated with the set of content items. In the above example, the normalization module 250 may access the achievement store 245 to retrieve information describing progress made by the online system 140 towards achieving the reach goal and the impression goal. In this example, if a reach count associated with the reach goal indicates that the set of content items has been presented to 200 different online system users and an impression count associated with the impression goal indicates that the content items have achieved 600 impressions, the normalization module 250 may compute the achievement ratio to have a value of 0.33 (i.e., 200/600). In alternative embodiments, the normalization module 250 may compute a normalized ratio in a different manner. In embodiments in which the online system 140 guarantees achievement of one or more types of goals that are different from those just described, the normalization module 250 may compute the normalized ratio based on the types of goals and on the progress made by the online system towards achieving the corresponding types of goals. For example, if a set of content items is associated with a click goal and a conversion goal, the normalization module 250 may compute a goal ratio to be a ratio of the conversion goal to the click goal and an achievement ratio to be a ratio of achieved conversions to achieved clicks. In this example, the normalization module 250 may then compute the normalized ratio to be a ratio of the achievement ratio to the goal ratio. In some embodiments, at least some of the ratios may be expressed as inverses of the ratios described in the above examples. For example, the normalization module 250 may compute the normalized ratio to be a ratio of the goal ratio to the achievement ratio. As an additional example, the normalization module 250 may compute the achievement ratio to be a ratio of the achieved impressions to the achieved reach and the goal ratio to be a ratio of the impression goal to the reach goal. The functionality of the normalization module 250 is further described below in conjunction with FIG. 3. In some embodiments, once the normalization module 250 has computed one or more normalized ratios, the graphing module 255 may plot (e.g., as shown in step 328 of FIG. 3) each normalized ratio as a function of time on a graph. For example, if the normalization module 250 computes a normalized ratio at a particular time, the graphing module 255 may plot a point on a two-dimensional graph, in which the point has a y-coordinate corresponding to the normalized ratio and an x-coordinate corresponding to the time at which the normalized ratio was computed. In this example, the graphing module 255 also may plot a point corresponding to each normalized ratio subsequently computed by the normalization module 250 on the same graph. In some embodiments, once the graphing module 255 has plotted multiple points on a graph corresponding to multiple normalized ratios as a function of time, the graphing module 255 also may generate a “trend line” that best fits the points. For example, the graphing module 255 may generate a trend line that best fits multiple points plotted on a graph that correspond to multiple normalized ratios, such that the trend line shows a general direction in which the normalized ratio seems to be heading over time. In embodiments in which a set of content items is associated with multiple goals, in addition to plotting one or more points corresponding to normalized ratios as a function of time
{'bibliographic_information': "{'country': 'US', 'doc-number': '15829866', 'kind': 'B2', 'date': '20171201', 'extension': '368', 'disclaimer_text': 'This patent is subject to a terminal disclaimer.', 'invention_title': 'Managing impression opportunities for a set of content items associated with goals to guarantee achievement of the goals within a specified period of time'}", 'source_file': './grant/2021/ipg210601.xml', 'source_data': 'USPTO-Grant'}