The preceding section have convered the most important extraneous variables and way to control them . in this final section ,we will look at another set of extraneous variables, context variables, that can influence the results of experiments. context variables are those that come about from procedures created by the environment, or context , of the research setting context variables represent subject recruitment ,selection .and assignment procedures ,as well as typical problems encountered in research on a university population. most are easily controlled with some forethought. we will cover two basic kinds of context variables ;those occurring when subjects select their own experiment and those produced when experimenters select their own subjects.