in terms of responsiveness and stability under a particular workload. It can also serve to investigate, measure, validate or verify other quality attributes of the system, such as scalability, reliability and resource usage. Load testing is primarily concerned with testing that the system can continue to operate under a specific load, whether that be large quantities of data or a large number of users. This is generally referred to as software scalability. The related load testing activity of when performed as a non-functional activity is often referred to as endurance testing. Volume testing is a way to test software functions even when certain components (for example a file or database) increase radically in size. Stress testing is a way to test reliability under unexpected or rare workloads. Stability testing (often referred to as load or endurance testing) checks to see if the software can continuously function well in or above an acceptable period. There is little agreement on what the specific goals of performance testing are. The terms load testing, performance testing, scalability testing, and volume testing, are often used interchangeably. Real-time software systems have strict timing constraints. To test if timing constraints are met, real-time testing is used. Usability testing Usability testing is to check if the user interface is easy to use and understand. It is concerned mainly with the use of the application. This is not a kind of testing that can be automated; actual human users are needed, being monitored by skilled UI designers. Accessibility testing Accessibility testing is done to ensure that the software is accessible to persons with disabilities. Some of the common web accessibility tests are Ensuring that the color contrast between the font and the background color is appropriate Font Size Alternate Texts for multimedia content Ability to use the system using the computer keyboard in addition to the mouse. Common Standards for compliance Americans with Disabilities Act of 1990 Section 508 Amendment to the Rehabilitation Act of 1973 Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C) Security testing Security testing is essential for software that processes confidential data to prevent system intrusion by hackers. The International Organization for Standardization (ISO) defines this as a "type of testing conducted to evaluate the degree to which a test item, and associated data and information, are protected so that unauthorised persons or systems cannot use, read or modify them, and authorized