Bias-Variance Tradeoff

Lead Author(s): Jeff Martin, MD

Bias-Variance Tradeoff

The tension is between trying to get the valid answer, or the least biased answer without having to take unnecessary penalties in precision. This is known as the bias-variance tradeoff, and the methodologic literature is inconsistent on the best way to handle this, mainly because there is no one correct approach. This leaves the investigator on his/her own. The only scientifically rigorous approach to this would be to create two lists of potential confounders prior to conducting the analysis.

A List of Potential Confounders

A-List: Those factors for which you will accept the adjusted result no matter how small the difference from the crude.

B List of Potential Confounders

B-List: Those factors for which you will accept the adjusted result only if it meaningfully differs from the crude (with some pre-specified difference, e.g., 10%).

Using the A List and the B List

Always putting all factors on A list may seem conservative, For some analyses, you may have no factors on A list. Recommendation: NOT many variables on A list

Adjusting or Ignoring B List Factors

Here are a few numerical examples showing when to adjust and when not to adjust for factors on the B list.


1) Column 1 is the crude estimate.

2) Columns 2 and 3 are the stratum-specific estimates.

3) Column 4 is the adjusted estimate, the weighted average of the stratum specific estimates.