Research has shown that perceived message effectiveness (PE) correlates reasonably well with indices of actual effectiveness, but little attention has been given to how to interpret mean PE. This article describes the problem of mean validity and presents a research design that can be used to address it. Participants (N = 195) viewed messages that advocated being screened for colorectal cancer. The results showed downward bias in PE among members of the non-target audience (persons younger than 50) and upward bias as the referent for the judgment became more abstract/distant (self vs. persons older than 50 vs. general). The need for more research on mean validity is discussed. For applied researchers, recommendations for preferred indices of PE are offered.