This is an example of data that follows the common effect model.
For each study, the blue circle gives the true result that we would find if each study could include everyone in the population.
Note that all the circles are in the same place because every study has the same “true” result, known as the “true effect,” of a particular treatment or intervention. Of course, we can’t know what the true effect is. We only know the results from each study.
The light blue diamonds describe the actual study results. Each study has a slightly different result, so the diamonds are in different locations even though they all describe the same “true effect.”
Each curve describes the range of possible study results. The closer the results are to the peak of the curve, the more likely they are to happen. The curves have different shapes because they describe different variances.
You’ll notice the curve is highest in the middle, where the true effect is.