α denotes the probability that an arbitrarily arriving customer order will be completely served from stock on hand, i.e. without an inventory-related waiting time (period service level): . In order to determine the safety stock that guarantees a target service level, the stationary probability distribution of the inventory on hand must be known. This version of α is also called ready rate. If an order cycle is considered as the standard period of reference, then α denotes the probability of no stockout within an order cycle which is equal to the proportion of all order cycles with no stockouts (cycle service level): This second definition, which is often used in operations management textbooks, is based on the idea of not running out of stock during the time between re-ordering and order arrival (the leadtime). That is, the probability of demand during that leadtime being less than or equal to the amount of stock you had left when you ordered. It assumes your reorder point is positive, that orders are in unit increments and inventory is monitored continuously so you cannot stock out prior to reordering. β service level (type 2) The β service level is a quantity-oriented performance measure describing the proportion of total demand within a reference period which is delivered without delay from stock on hand: This is equal to the probability that an arbitrary demand unit is delivered without delay. This approach usually involves calculating a loss integral, whose values are tabulated for the normal distribution. Because, contrary to the variations of the service level, the service level does not only reflect the stockout event but also the amount backordered, it is widely used in industrial practice. Also, by the definitions, comparing service levels we have whenever the probability of zero demand equals 0. γ service level The γ service level, a time- and quantity-related performance criterion, serves to reflect not only the amount of backorders but also the waiting times of the demands backordered. The γ service level is defined as follows: The γ service level is rarely used in industrial practice. Service rate In business, service rate is a performance metric used to measure the customer service in a supply organization. One example of a service rate measures the number of units filled as a percentage of the total ordered and is known as fill rate. If customer orders total 1000 units, and you can