Intermediary Variable

Lead Author(s):

Intermediary Variable Is Not a Confounder

As seen in the diagram below a variable that is an intermediate step in the causal path under study

DESPITE BEING ASSOCIATED WITH BOTH EXPOSURE AND OUTCOME, Often na\xEFve investigators think of an intermediary variable as a confounder - You might be tempted to adjust for it if you naively looked at basic requirements of a confounding factor (associated with disease and exposure).

The issue with confounding is not just that you want to adjust for factors associated with the exposure and the outcome Therefore, if you believe E caused D through some factor I and this is the pathway you are interested in studying,

Is It an Intermediary Variable or a Confounder?

How do you determine if it is and intermediary variable or a confounder?

(1) Ask: What is your research question?

(2) Ask: What is the pathway under investigation?

What Is Your Research Question?

An excellent example of determining your research question can be found in Taylor's Study of the CCR5 receptor in HIV.

To determine if CD4 count is an intermediary variable we ask: To determine if CD4 count is a confounding variable we ask:

What Is the Pathway Under Investigation?

An excellent example of determining the pathway under investigation can be found in Pathway Under Investigation - CAD.

If you do not care about the exact pathway or you are only thinking about one pathway by which exercise can influence the development of heart disease, and HDL was right in the middle of this pathway, If the pathway you are interested in pathway other than HDL,

Third Variable: Decision Tree - Intermediary Variable

The first decision you have to make is a conceptual one based upon your knowledge of the biological or behavioral system and the research question at hand. 0415_8yes_interm.JPG