Researchers undertook a meta-analysis of the effects of home monitoring on blood pressure. Randomised controlled trials were included if participants had essential hypertension and were trying to achieve blood pressure targets. The participants who did home blood pressure monitoring used ambulatory monitors. The control intervention was standard blood pressure monitoring in the healthcare system. In total, 13 randomised controlled trials were identified, with length of intervention ranging from two months to three years.1 Primary outcome measures included the difference between treatment arms in mean systolic blood pressure. It was reported that home blood pressure monitoring resulted in lower systolic blood pressure than standard monitoring, with an overall mean difference of 4.2 mm Hg (95% confidence interval 1.5 to 6.9 mm Hg). A funnel plot was presented (figure⇓). The funnel plot displayed asymmetry, and Egger’s test gave a statistically significant result (P=0.038). Funnel plot for the meta-analysis of the effects of home blood pressure monitoring on systolic blood pressure in comparison with standard monitoring Which of the following may have led to asymmetry in the funnel plot?