Since it should be made in the natural environment, observation provides data impossible to get in the usability laboratory. For instance, when testing a public telephone, people's reaction will sure be different in a controlled than in a public environment. There is no experimental or other manipulation. Useful when there are behaviours or situations on which subjects are unable to report with accuracy, such as, e.g., with children, or non-verbal behavior. Disadvantages Very costly and really difficult to carry out.
Validity depends heavily on the categorization process.
It can be difficult to know the reasons why particular behaviors are made.