Name the confounder: shoe size and reading ability

Question

A researcher finds that among children at a primary school, those with larger shoe sizes tend to score higher on reading tests. Someone jokes that stretching your feet will make you read better.

What third variable drives both shoe size and reading ability, producing this correlation? Name it and explain.

Accepted Answer

The confounder is the child's age. Older children have bigger feet (larger shoe size) and have had more schooling and development, so they read better. Age is a common cause of both variables; shoe size and reading ability have no direct link at all. The correlation is entirely a byproduct of mixing children of different ages together. The tell. Hold age fixed and the correlation vanishes: among seven-year-olds, the ones with slightly bigger feet do not read any better. That is the signature of a confounded relationship, it disappears once you condition on the common cause. Takeaway. A strong…

Name the confounder: shoe size and reading ability

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