In the first hour, the subject averages 150 responses per minute. In the second hour, he averages 120 responses per minute, by the fourth hour, he is down to 100 per minute. Assume that each response is correct, i.e., no mistakes are made by the subject. The job varies in that sometimes it involves sorting things into two bins, four bins, or six bins. The same person working at the same task for a year would become practiced and faster.
Experience leads to shorter reaction times and better performance.
If we change the bins, it would have a damaging effect on the performance of the operator and he would make many mistakes early on. Over time, the performance would get to be as good as it was originally.