A meta-analysis of several trials on the effect of cocoa intake on systolic blood pressure (SBP) revealed the following results: All the trials evaluated the difference in SBP at 2 weeks. Based on the data, which of the following is the most appropriate conclusion?

Question

A meta-analysis of several trials on the effect of cocoa intake on systolic blood pressure (SBP) revealed the following results:  All the trials evaluated the difference in SBP at 2 weeks.  Based on the data, which of the following is the most appropriate conclusion?

Quizplus · Accepted Answer

A meta-analysis groups the results of several trials (ideally, high-quality randomized controlled trials) to increase statistical power and provide an overall estimate of the effect of an exposure (eg, cocoa intake) on an outcome (eg, systolic blood pressure [SBP]). The individual results of the trials are assessed and weighted to obtain a pooled effect estimate. In this example, the meta-analysis showed an overall change in mean SBP of −2.2 mm Hg (ie, decrease in SBP) in the cocoa intake group compared to the control group. The 95% confidence interval (CI) was −2.7 to −1.3 mm Hg, indicating a statistically significant difference because the CI does not include the null value (explained below). CIs give a range of plausible values for an unknown parameter (eg, difference between 2 mean SBPs) based on results from a sample. If the CI does not include the null value, then the result is statistically significant; if it crosses the null value, then the result is not statistically significant. All CIs have a null value, but the null value is not the same for all CIs. For an odds ratio or a relative risk (RR), the null value is 1 because these statistics are ratios (ie, RR = 1 represents no difference in risk between the groups). However, if the parameter of interest is a difference (eg, difference in mean SBP between cocoa intake and control groups), then the null value is 0 because that represents no difference between the groups. In this case, the 95% CI of [−2.7, −1.3] does not include the null value of 0 and therefore the result is statistically significant. In summary, cocoa intake was associated with a statistically significant decrease in SBP. (Choices A and E) The overall mean change in SBP was negative at −2.2 mm Hg, reflecting a lower SBP in the cocoa group compared to the control group. The change was statistically significant. (Choice B) Although there was a statistically significant decrease in SBP in the cocoa intake group, the clinical significance may be limited as the absolute SBP decrease is only about 2 mm Hg. Furthermore, the trials measured SBP at 2 weeks, so it is possible that the effect of cocoa intake on SBP was short-lived. Finally, no information is provided regarding adverse effects of cocoa intake. (Choice D) Given that the CI crossed the null value in studies 2 and 7, there was no statistically significant difference between the groups in these studies. Educational objective: A meta-analysis groups results of several trials to increase statistical power and provide an overall pooled effect estimate.

A Meta-Analysis of Several Trials on the Effect of Cocoa