The result of a usability test depends very much on the type of test persons used. Novice This is the person who has no prior experience with this type of apparatus. This type of test person will have the most problems finding out how to use the thing, and hence find many usability errors. This is a person who has a lot of experience using a similar product. The experienced user will try the advanced features and will know where to look for problems.
Old people learn more slowly and their senses and motor skills are less efficient.
For example, my old mom has difficulties double-clicking because the mouse doesn't tolerate even the smallest movement between the two clicks. Handicapped user Letting people with various handicaps test your device can be quite revealing. If test persons with the right handicaps are not easily found then you may study various guidelines for making things accessible to handicapped persons. See the list of references. Child Children are curious and adventurous.