6.2 Are confounding variables only found with categorical data?
The problem of confounding variables is not restricted to examples involving only categorical variables.
Scenario
The data presented below led a newspaper in the US to argue that US doctors were growing in number but not in pay (Barnett, 1994).
Two possible interpretations of this data are:
1. An increase in the number of doctors in the US between 1970 and 1982 is lowering the value of individual doctors so that their average earnings are lower
2. In 1982 there are more inexperienced doctors than in 1970 so their average salary is lower
Test your knowledge
Question
Identify the variable presented as a confounding variable in interpretation 2 that was not part of the explanation presented in interpretation 1.
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