Relative ingroup prototypicality (RIP) is an important concept in the ingroup projection model (IPM) of social discrimination and tolerance. This paper reviews measures of RIP currently in use and critically examines how the notion of RIP is captured by statistical tests treating RIP as a single variable. It is concluded that composite measures of RIP imply multiple statistical hypotheses that have previously been confounded. The value of an alternative multiple regression approach is illustrated in a study testing the hypothesis of a negative relationship between RIP and outgroup attitudes. Results based on the conventional univariate analyses would have confirmed or disconfirmed the hypothesis depending on the scoring method. In contrast, the multiple regression approach described in this paper resolves this ambiguity by suggesting that only outgroup prototypicality may be necessary to predict outgroup attitudes.