The Forest Plot Trilogy - a gripping thriller concludes

Forest plots, funnel plots - and what's with the mysterious diamond symbol, lurking like a secret sign, in meta-analyses? Meta-analysis is a statistical technique for combining the results of studies. It is often used in systematic reviews (and in non-systematic reviews, too).

A forest plot is a graphical way of presenting the results of each individual study and the combined result. The diamond is one way of showing that combined result. Here's a representation of a forest plot, with 4 trials (a line for each). The 4th trial finds the treatment better than what it's compared to: the other 3 had equivocal results because they're crossing the vertical line of no effect.

A funnel plot is one way of exploring for publication bias: whether or not there may be unpublished studies. Funnel plots can look kind of like the sketches below. The first shows a pretty normal distribution of studies - each blob is a study. It's roughly symmetrical: small under-powered studies spread around, with both positive and negative results.

This second one is asymmetrical or lopsided, suggesting there might be some studies that didn't show the treatment works - but they weren't published:

Gaping hole where negative studies should be

More on this: 5 Key Things to Know About Meta-Analysis and Another 5 Things to Know About Meta-Analysis.

(This post uses snapshots from slides I'll be using to explain systematic reviews at the 2012 NIH Medicine in the Medicine course that's starting this weekend. It's several days of in-depth training in evidence and statistics for journalists. This year it's being held at Potomac, just near Washington. And here's a post on the start of the course that I wrote for Scientific American online.)

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