Talking about bias is so pre-millennial. The kool kids are more interested in metabias. Metabias, by definition, only rears its ugly head when groups of studies are examined. Meta-epidemiological studies (e.g. meta-analyses) can uncover risk factors for bias that don’t seem to be associated with a process active in an individual study. That’s metabias! We’ve blogged about several specific forms of metabias before, including publication bias and citation bias. A new form of metabias is described in this week’s Annals of Internal Medicine: it turns out that single-center trials consistently report larger treatment effects than multicenter trials, even after controlling for sample size and other factors. It’s not clear why this is the case, nor is it possible to determine which study type (single-center or multicenter) gets closer to the “truth” (though I strongly suspect the answer is multicenter). Given that many HAI prevention studies happen to be single center studies, this problem bears further scrutiny…..and a willingness to fund larger (more expensive) multicenter trials.