Figure 2

True and observed ROC curves for a hypothetical example where bias changes the scientific conclusion. The parameters for this example were chosen to illustrate a case where paired screening trial bias may cause an incorrect scientific conclusion. The incorrect conclusion occurs because for participants with disease, one screening test leads to a higher chance of recalls than the other screening test. The chance of recall for Test 2 for a participant who had disease was 34%, while for Test 1 it was 8%. For this example, we fixed the disease rate, r = 0.01; the chance that participants with disease would experience signs and symptoms within the year of follow-up, ψ = 0.1; the variance, σ ² = 1. The means of the distributions of test results for cases for Test 1 and Test 2 were 2.1 and 1.1, respectively, and the means for non-cases for Test 1 and Test 2 were 1.6 and 0.35. The correlation between test scores for cases was fixed at 0.1, as was the correlation for non-cases. All test scores above 2.5 on either test, or participants who had signs or symptoms had an infallible secondary test to determine disease status. For participants with scores below 2.5 on both tests, a less sensitive method was used to approximate disease status.