The relationship between the log risk ratio for postoperative nausea and vomiting from nitrous oxide (lnRR PONV_N2O) and duration of exposure to nitrous oxide (N₂O), as a bubble plot. The meta-regression line of best fit (linear prediction) and upper and lower 95% CIs are shown. Bubble size is inversely proportional to the standard error of the log risk ratio in each study.