How could you differentiate between people who use their computers least as being the most productive (they get everything done in the shortest time and then do something else), or the least productive (they do the least work)? Or the people who use their computers most as the most productive (they get the most done), or least productive (they spend more time on work activities but still fail to complete things).

Only measuring how much time you spend on something isn't enough to measure your productivity.