High Quality Content by WIKIPEDIA articles! In probability and statistics, Simpson`s paradox (or the Yule–Simpson effect) is a paradox in which a correlation present in different groups is reversed when the groups are combined. This result is often encountered in social-science and medical-science statistics, and it occurs when frequency data are hastily given causal interpretations. Simpson`s Paradox disappears when causal relations are brought into consideration (see Implications to decision making).