Meta-analyses: what is heterogeneity?

Researchers undertook a meta-analysis to evaluate the effectiveness of multifactorial assessment and intervention programmes in preventing falls and injuries among older people. Randomised or quasi-randomised trials that evaluated interventions to prevent falls and injuries were included. The intervention had to be delivered to individual patients, not at a community or population level. It also had to be service based in an emergency department, primary care, or the community. Control groups could receive standard care or no fall prevention. The outcomes included the number of fallers and fall related injuries.1 In total 19 trials were identified. Of these, eight reported fall related injuries. When combined across trials, the risk for fall related injuries was reduced after the intervention compared with the control, but not significantly (relative risk 0.90, 95% confidence interval 0.68 to 1.20). Tests of statistical heterogeneity for the meta-analysis of fall related injuries gave the following results: χ2=15.77, degrees of freedom=7, P=0.03 (Cochran's Q test), I2=55.6% (Higgins's I2 test statistic). Subgroup analyses using a test of interaction based on Cochran's Q test were subsequently performed. The resulting P values were: P=0.75 for site of delivery (hospital v community); P=0.75 for whether a doctor was included in the team (yes v no); and P=0.52 for whether trial participants had been selected because they were at high risk of falls (yes v no). The study concluded that there was limited evidence that multifactorial fall prevention programmes in primary care, community, or emergency care settings were effective in reducing the number of fallers or fall related injuries. Which of the following statements, if any, are true for the meta-analysis of fall related injuries?