According to Robert Selman's theory of friendship development, the first stage of friendship is based on:

In the context of trait approaches to personality, ________ is a statistical method of identifying patterns among a large number of variables and combining them into more fundamental groupings.

Fill in the blank(s) with the appropriate word(s).

Mail surveys are particularly vulnerable to a problem that arises when not all respondents complete and return the survey. This problem is called

A. selection bias. B. reactivity. C. low response rate. D. social undesirability.