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What is a Forest Plot?

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University Course Reader · STEM

Every trial in the review sat on its own row, and only the diamond at the foot of the forest plot cleared the line of no difference.

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Overview

A forest plot is a chart that shows, in one picture, what each study on a question found and what they all add up to. Every study gathered gets a row, with its result and its margin of doubt. A line down the middle marks the result that would mean no effect at all. A row crossing that line means the study on its own could not tell. The diamond at the foot is the combined answer, built from every row at once.
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Overview

A forest plot squeezes one straight answer out of a stack of wishy-washy trials. Every trial that made the cut earns a bar showing its verdict plus however much wiggle room it admits to, usually 95% worth. A bar touching the centre stripe, the stripe that means nothing happened, is a trial shrugging. The diamond underneath is the payoff, where all the shrugging fuses into one call that actually commits. 😎

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Detail

A forest plot is the chart researchers draw when they combine studies on one question into a meta-analysis, so the gathered evidence and its combined answer can be seen at once. It holds the studies the reviewers could find and judged sound, and each of those gets a row. A square sits where the study landed, and the horizontal line through it is the confidence interval, usually 95%, the range that study could narrow the answer to. A vertical line marks the spot that would mean the treatment did nothing at all. The part readers get wrong is what a crossing row means. It does not mean the study found no effect. It means the study was too small to tell a real effect from none at all, an ordinary result for a single trial. The diamond underneath combines every row, with the larger squares counting for more. It is normally tighter than any row above it, because more data narrows the range, so seven rows that each straddled the line can produce one diamond that clears it. What the chart cannot show is whether studies are missing from it, since trials sharing one flaw still yield a tight confident diamond. Checking for what went missing needs a different chart, the funnel plot.
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Detail

People hunt down the trials on one topic, as many as they can actually find, and the picture that sums up the pile is a forest plot. Read it bottom first. The diamond is the answer, everything over it is the working, and people get this backwards constantly. They scan the bars, count how many lean the good way, and announce the evidence is mixed. Nope. A bar crossing the middle is a trial saying I could not tell, not a trial saying no. Loads of those can fuse into a diamond that lands firmly on one side, which is the entire reason anybody bothers drawing the thing. Bar length is the tell for size, so a 50-person run gets a long stringy bar, a 5,000-person run gets a stubby one, and the stubby ones drag the diamond hardest. The name is not deep either. Somebody decided it looked like a forest of lines, and for a while people called it a blobbogram instead, which is objectively better. However you say it, read the diamond first. 😎

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Analogy

Six neighbours test whether the back lane really beats the main road. Each times their trips for a fortnight and comes back shrugging. Saved about a minute, maybe, some mornings it was slower, nobody is sure. Put all the trips together and there is finally enough to go on, and the shortcut saves about ninety seconds. The neighbour who logged 40 trips sways the verdict more than the one who logged 4, since more trips make a steadier average. A forest plot is this picture. Each row is one shrugging neighbour, and the verdict at the foot is every trip counted at once.
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Analogy

Six mates each flip the identical coin ten times to settle an argument about whether it is bent. Everyone lands on 6 and 4, or 7 and 3, and everyone gives up, because ten throws prove absolutely nothing. Then you tip all sixty results into a heap and it reads 40 to 20. Suddenly the argument is over. Not one person held enough evidence alone, and the heap held plenty of it. Identical coin, identical throws, and the only new ingredient was somebody bothering to add them up. 😎
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AI explanations may contain errors · Not professional advice

Formal definition — The same term, explained the usual way

A forest plot is a graphical display of the results of a meta-analysis, in which each contributing study is represented by a marker at its estimated effect and a horizontal line spanning its confidence interval, conventionally 95%. The marker's area is commonly drawn in proportion to the study's weight in the analysis. A vertical reference line denotes the null value, and the summary estimate for all studies combined is displayed at the foot of the plot as a diamond whose lateral extremities represent its confidence interval. Statistical significance at the stated level is indicated where the diamond does not intersect the reference line.

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