Simpson's Paradox (A.K.A. Yule-Simpson Effect) Postcard

Statisticians who work in the social sciences know about "Simpson's Paradox (A.K.A. Yule-Simpson Effect)", 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 is often found when frequency data are given causal interpretations. The practical significance of Simpson's paradox surfaces in decision making situations where it poses the following dilemma: Which data should we consult in choosing an action, the aggregated or the partitioned? If you're a die-hard statistician, you'll enjoy any of these Simpson's paradox gifts featuring a graph for continuous data. Notice how a positive trend appears for two separate groups (blue and red), yet a negative trend (black, dashed) appears when the data are combined.