User trials
In user trials the users are not necessarily observed during the the assessment. The aim is to encourage more 'natural' interaction with the product, to remove any changes in behaviour that may arise from the user being observed. Feedback from the trials would be initiated by the user or else obtained through interviews or questionnaires.
However, their weaknesses include a strong dependence on how well the users have understood the purpose and procedure of the trial and their ability to communicate their feedback to the assessor. This is especially important when considering products designed for users with learning or communication impairments. Again, like user observations, user trials can only really be used when there is a working prototype available to assess.