Scientific and statistical errors should have been picked up in peer review

EDITOR—The paper by Henkel et al contains several scientific and statistical errors, which should have been identified and eliminated by the BMJ's peer review process before publication.1 The study is described as a cohort study but is a cross sectional study.

The general health questionnaire is inappropriate as a screening test for depression and should not be compared with a gold standard test that is specific for depression. It is a screening test for the “common mental disorders” that often occur in primary care,2 with a good sensitivity but poor specificity for depression.

Henkel et al choose sensitivity and negative predictive value as measures of how well their tests perform. Sensitivity should be paired with specificity. Sensitivity and negative predictive value are not independent.

They do not make clear their primary hypothesis or explain what questions they are seeking to examine in their analysis. With so many different measures that they could compare, a plan of analysis is essential. They could, for example, start with an omnibus test for sensitivity, such as Cochran's Q.3

Henkel et al use one sided statistical tests but should have used two sided tests. Exact P values or confidence intervals should have been given. Fleiss gives the procedure for confidence intervals in matched pairs.3

They have carried out multiple statistical tests without making a correction. To maintain the overall probability of obtaining a type I error at 0.05, a Bonferroni or other correction is usually made to the significance level of the test.

Henkel et …