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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.

• Factors you know or believe are confounders.
• Example: Age, when looking at the association between hypertension and coronary artery disease.

## 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%).
• Factors you are less sure about causing confounding.
• These might be factors which you think may be confounders but you are not sure about.

## Using the A List and the B List

Always putting all factors on A list may seem conservative,
• but not necessarily the right thing to do in that there may be a penalty in statistical precision.
For some analyses, you may have no factors on A list.
• For other analyses, you may have no factors on B list.
Recommendation: NOT many variables on A list
• Put just about everything on the B 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.

• If the crude is 4.0 and the adjusted is 2, you would always report the adjusted estimate.
• If the crude is 4.0 and the adjusted is 1.1, you would definitely report the adjusted.
• If the crude is 0.2 and the adjusted is 0.8, you would definitely report the adjusted.
• If the crude is 4.0 and the adjusted is 4.1, you would not adjust. You would ignore this in favor of the crude estimate.
• If the crude is 1.9 and the adjusted is 1.8, you would not adjust.

Topic revision: r7 - 18 Jan 2010 - 16:53:26 - MaryB?

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