This paper is meant to highlight the occurrence of Simpson's Paradox when using aggregated data obtained from two IEA studies in Israel, while ignoring the effect of a powerful intervening variable in the local context--the ethnicity factor. It will demonstrate faulty conclusions regarding either the absence of relationships between a contextual variable and achievement--when such relationships do exist, or conclusions on the existence of such relationships--when in reality they do not. Our intention in writing this paper is to draw the attention of our fellow researchers to similar faulty inferences they might come across when analyzing their local database within the scope of international comparative studies. (Contains 6 tables.)