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Example of Misclassification Bias

Non-Differential Exposure - Imperfect Sensitivity

Lead Author(s): Jeff Martin, MD

Measurement bias is affected by imperfect sensitivity of exposure variables.

Imperfect Sensitivity of Exposure Variables

0519_non_diff_exp.JPG
In the diagram above the source population is on the top left and our study sample is on the bottom right.
  • We use our study sample to make inferences about the source population.

Study - Alcohol Abuse

Consider as an example of a case-control study of some disease and the exposure under study is alcohol abuse.
  • Let's consider that among all true alcohol abusers our measurement, which is self-report, is only able to identify some fraction.
  • In other words, we aren't able to get some alcohol abusers to admit that they are abusers.
  • Hence, our measurement of alcohol use is insensitive. We depict this by showing an arrow going from the exposure cell to the unexposed cell.
  • This is lack of complete sensitivity; in other words, we are misclassifying some alcohol users as non-users.

Misclassification of Exposure

  • This is called misclassification of exposure.
  • Because the misclassification of exposure is occurring equally among the cases and controls, we call this non-differential with respect to disease.

Non-Differential Misclassification of Exposure

  • Overall, this is called non-differential misclassification of exposure.
Again, some truly exposed persons are misclassified as unexposed.
  • Because this happens equally among diseased and non-diseased persons, it is called non-differential misclassification.

Imperfect Sensitivity - Bias the OR towards the Null Hypothesis

0114_eg_sens.JPG

In the top panel above is the truth where there is no misclassification of exposure -

What happens in the presence of 70% sensitivity in exposure classification?
  • That means that 30% of truly exposed cases or 15 of 50 are instead classified as unexposed.
  • Also, 30% of the 20 exposed controls are falsely classified as unexposed, or 6 persons.
  • The bottom panel therefore shows what happens and you can see that the OR is now attenuated to 3.3.

Bias Towards the Null Hypothesis

This illustrates the effect of non-differential misclassification of exposure in the presence of 2 exposure categories -


Topic revision: r6 - 03 Feb 2012 - 17:10:58 - PeterBacchetti
 

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