Learn By Doing – Causation and Lurking Variables

Recall that an instance of Simpson’s paradox occurs whenever including a lurking variable causes you to rethink the direction of an association. In this activity, we will reinforce our understanding of Simpson’s paradox.

The following scatterplot displays the relationship between two quantitative variables, X and Y:

This graphical display indicates that the overall relationship between X and Y is negative.

In each of the four labeled scatterplots below, we included a different lurking variable, which separates the data points into two groups, blue points and red points. Your task is to look at all four displays and decide in which case including the lurking variable leads to an instance of Simpson’s paradox.

http://phhp-faculty-cantrell.sites.medinfo.ufl.edu/files/2013/01/qzLBD02024.swf

Hint

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This document is linked from Causation.