How to read a forest plot in a meta-analysis

Researchers undertook a meta-analysis of the effect of treating Helicobacter pylori with eradication therapy on the subsequent occurrence of gastric cancer. Randomised controlled trials were included if the intervention consisted of seven days or more of eradication therapy, and if the control treatment was placebo or no treatment. Participants were adults who tested positive for H pylori. They were otherwise healthy and asymptomatic at baseline and were followed for two or more years. The primary outcome was the diagnosis of gastric cancer.1

Six randomised controlled trials were eligible for inclusion. The results of the meta-analysis comparing the intervention with control treatment in the occurrence of gastric cancer were presented in a forest plot (figure⇓). Fifty one (1.6%) gastric cancers occurred in 3294 participants who received eradication therapy compared with 76 (2.4%) in 3203 control subjects (relative risk 0.66, 95% confidence interval 0.46 to 0.95).

Which of the following statements, if any, are true?

• a) All six trials showed a significant difference between eradication therapy and control treatment in the risk of gastric cancer

• b) The forest plot is drawn on a logarithmic scale

• c) A risk ratio greater than 1.0 indicates an increased risk of gastric cancer with the control treatment compared with eradication therapy

• d) The total overall estimate of the population risk ratio indicated that eradication therapy led to a 34% lower risk of gastric cancer compared with the control treatment

• e) Significant heterogeneity existed between the sample estimates of the population risk ratio of gastric cancer