Fig. 2

I stands for true intervention status, I* for measured intervention status, O for true outcome status, O* for measured outcome status, and U for a third (usually unmeasured) variable. The arrow with question mark stands for the causal question (effect) of interest. The red arrow signifies that Intervention I affects measured outcome O*, the green arrow that Outcome O affects measured intervention I*, the purple arrow that a third factor U affects measured intervention I*, and the blue arrow that a third factor U affects measured intervention I* and outcome O*. Boxed nodes indicate variables in the analysis