People are not at all times very dependable informants, even when they try to be trustworthy and enlighten what they believe is the fact. Since people cannot always be trusted to reliably interpret and report their actions, what people tell can be supplemented by watching what they do or obtaining comparatively objective measures of how people behave in work situations. In light of this, Sommerville asserts that observation is one of the most efficient methods for understanding information systems requirements. With this, it is likely to either participate in or watch a person perform activities to learn about the system .
This technique is mainly useful when the validity of data collected through other methods is in question or when the complexity of certain aspects of the system prevents a clear explanation by the end-user.
For example, checking the electronic mail records of an employee may be able to tell you how much he uses electronic mail. An employee might tell you that he is overwhelmed with e-mail messages and that he spends a lot of time responding to those messages. However, if you check electronic mail records, you might find that the employee receives few messages. Another example might be the information obtained from a manager. A manager might tell you that she works for a long and she is careful with a tight schedule in controlling the pace of work. Nevertheless, you can find that the managerâ€™s day is actually punctuated by many interruptions including phone calls and visits.