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Why you should be suspicious of subgroups: A new addition to our toolkit

Kevin Lomangino is the managing editor of He tweets as @KLomangino.

Our toolkit is chock full of resources designed to help journalists and the public think smarter about health care.

And we just added a brand new primer: 3 reasons why you should be suspicious of study ‘subgroup’ results.

Why do we need to know about subgroups?

Because they’re often touted in news stories and PR releases as evidence that a treatment is effective. But experts warn that these analyses — which involve looking at a small group within a larger study — are subject to serious, sometimes fatal limitations.

And media messages about subgroups rarely explain why these results may be misleading.

Excerpts from the primer:

  • “Because they’re smaller than the overall study, subgroups are more likely to harbor subtle imbalances that can skew the findings.”
  • “Subgroups have a lousy track record of identifying legitimate health benefits. In a recent analysis of clinical trials that included a claim from a subgroup, researchers found that only 5 out of the 46 results they looked at were ever tested in a follow-up study. None of the 46 subgroup claims held up in the subsequent research.”
  • “Another problem with subgroups is the higher likelihood of false-positive findings. Simply put, the more subgroups you look at, the greater the risk that you’ll find a positive result that’s attributable to random chance rather than the intervention.”

Researchers who study this issue say, “Subgroup analyses have historically misinformed as much as they have informed.”

I hope you’ll read the entire primer and find out why.

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