The Volcano Plot tab, which is created when either of the inverse variance methods for combining p-values has been selected as the Method to Use and an Effect ID Variable has been specified, is shown below:
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When an inverse variance method has been used to combine p-values, a scatter plot shows the meta-analysis p-values and estimates of the overall effect. The y-axis variable of this scatter plot is the meta-analysis p-value, converted to the -log or -log10 scale if selected in the process dialog. The x-axis plots the estimate of the overall effect estimate for each effect, calculated by a weighted average of the individual study effects, where effect estimates can be regression coefficients or log odds ratios for example.
A horizontal reference line is drawn as a red, dashed line at the significance level that was specified. For -log or -log10-converted p-values, effects above this line are significant; for p-values on the original scale, effects below the line are significant.