Some time ago I wrote a post complaining that the New England Journal of Medicine was promoting testing of baseline characteristics in randomised trials, in their Instructions for Authors (post is here). After I wrote that I wondered if I was being fair; it could possibly refer to comparisons of outcomes, not baseline characteristics.
The good news is that NEJM have updated their instructions and clarified this. Here’s what it says now:
For tables comparing treatment groups at baseline in a randomized trial (usually the first table in the manuscript), significant differences between or among groups (i.e., P<0.05) should be identified in a table footnote and the P value should be provided in the format specified above.
So the bad news is that they are unambiguously telling people to include significance tests for basseline characteristics. Perhaps they should read CONSORT:
“Unfortunately significance tests of baseline differences are still common…. Such significance tests assess the probability that observed baseline differences could have occurred by chance; however, we already know that any differences are caused by chance. Tests of baseline differences are not necessarily wrong, just illogical. Such hypothesis testing is superfluous and can mislead investigators and their readers.”
Pretty amazing that they don’t get this really.
PS I should have included a link to the NEJM author instructions here. It’s in Guidelines for Statistical Methods, 12th (second last) bullet point. Amusingly, the last point says you have to provide all the information specified in the CONSORT checklist.