Wednesday, July 13, 2011

Gettin’ “meta” with it!

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.


  1. One of the reasons that single center HAI-prevention studies might report larger treatment effect is that they are typically uncontrolled quasi studies. Uncontrolled studies typically report higher effect sizes. I also think negative single-center studies are just swept under the rug, while large multicenter studies that are negative are more often published (e.g. STAR-ICU study).  This is publication bias, so the biases work hand-in-hand.
    Another example of a combined bias in single center HAI prevention studies is selection-regression where the intervention unit might be selected because that is where an outbreak is occurring and the control unit would be selected more at random.  Thus, the selected intervention unit might be subject to regression to the mean. This wouldn't be the case (typically) in individual person randomized RCTs as discussed in the Annals paper.

  2. Talking about meta-analysis, you should check out today's Annals of Internal Medicine issue: "July Effect" on patient's outcomes. They found higher mortality and longer length of stay among patients admitted at the beginning of the academic year. Their findings do seem to make sense though...despite being a meta-analysis...