Martin is interested in the effects of different kinds of instruction on videogame performance. Martin asks 30 college freshmen to each play 1 hour of Ratchet and Klank. Participants are randomly assigned to one of three instruction groups: (1) complete the tasks as quickly as possible, (2) conserve as much health as possible (i. e., play more carefully), or (3) find gold bolts (worth lots of money in equipment and ammunition). Obviously, even in a single instruction group, not all players will obtain the same final score. These differences in an instruction group reflect: