We wish to test for both true positive instances and true negative instances. As tests are done separately for each diseases with the device, should we consider it as 3 separate tests or should it be considered as single test for the purpose of sample size calculation?
If you want to validate the device for all 3 diseases in one step, then you should also consider this in the sample size calculation. You might want to look for multiple co-primary endpoints.
Best Matthias
Hello Smitha,
I'd suggest evaluating the sample size needed to evaluate sensitivity and specificity for each individual disease. If your presumptions (or pilot data) suggest that the three disease scenarios would yield different requisite Ns, then go with that which yields the largest N.
Your reporting can then focus on the relative success of the indicator measure for each of the three diseases (with the very likely possibility that any relevant measure, including AUC could well be different across diseases).
If, in practice, clinicians would routinely make simultaneous diagnoses for cases on this specific set of three diseases (as might be the case for evaluating whether a patient has an STD, for example), then you would be wise to also consider Matthias Kohl 's suggestion about an omnibus test.
Good luck with your work.
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