Overadjustment bias and unnecessary adjustment in epidemiologic studies
- PMID: 19525685
- PMCID: PMC2744485
- DOI: 10.1097/EDE.0b013e3181a819a1
Overadjustment bias and unnecessary adjustment in epidemiologic studies
Abstract
Overadjustment is defined inconsistently. This term is meant to describe control (eg, by regression adjustment, stratification, or restriction) for a variable that either increases net bias or decreases precision without affecting bias. We define overadjustment bias as control for an intermediate variable (or a descending proxy for an intermediate variable) on a causal path from exposure to outcome. We define unnecessary adjustment as control for a variable that does not affect bias of the causal relation between exposure and outcome but may affect its precision. We use causal diagrams and an empirical example (the effect of maternal smoking on neonatal mortality) to illustrate and clarify the definition of overadjustment bias, and to distinguish overadjustment bias from unnecessary adjustment. Using simulations, we quantify the amount of bias associated with overadjustment. Moreover, we show that this bias is based on a different causal structure from confounding or selection biases. Overadjustment bias is not a finite sample bias, while inefficiencies due to control for unnecessary variables are a function of sample size.
Figures
![FIGURE 1](https://www.ncbi.nlm.nih.gov/pmc/articles/instance/2744485/bin/nihms133589f1.gif)
![FIGURE 2](https://www.ncbi.nlm.nih.gov/pmc/articles/instance/2744485/bin/nihms133589f2.gif)
Comment in
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On the relative nature of overadjustment and unnecessary adjustment.Epidemiology. 2009 Jul;20(4):496-9. doi: 10.1097/EDE.0b013e3181a82f12. Epidemiology. 2009. PMID: 19525686 No abstract available.
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