Using data from multiple informants has long been considered best practice in education. However, multiple informants often disagree on similar constructs, complicating decision-making. Polynomial regression and response-surface analysis (PRA) is often used to test the congruence effect between multiple informants on an outcome. However, PRA assumes the congruence effect on the outcome is homogeneous across individuals. To investigate unknown heterogeneity in the congruence effect, we introduce polynomial regression mixture modeling (PRMix). We demonstrate how PRMix enhances congruence research by allowing researchers to investigate individual differences in the congruence effect. The differential congruence effects between latent classes are illustrated with response-surface plots. We provide practical suggestions for using PRMix and discuss future research directions in methodology.