EXAMPLE 9.12

Do we eat too much salt? Evidence from a variety of sources suggests that diets high in salt are associated with risks to human health. To investigate the relationship between salt intake and stroke, information from 14 studies was combined in a meta-analysis.⁴ Subjects were classified based on the amount of salt in their normal diet. They were followed for several years and then classified according to whether or not they had developed cardiovascular disease (CVD). A total of 104,933 subjects were studied, and 5161 of them developed CVD. Here are the data from one of the studies:⁵

Let’s look at the relative risk for this study. We first find the proportion of subjects who developed CVD in each group. For the subjects with a low salt intake, the proportion who developed CVD is

or 75 per thousand; for the high-salt group, the proportion is

or 90 per thousand. We can now compute the relative risk as the ratio of these two proportions. We choose to put the high-salt group in the numerator. The relative risk is

Relative risk greater than 1 means that the high-salt group developed more CVD than the low-salt group. For this study, the association is not statistically significant.

543

When the data from all 14 studies were combined, the relative risk was reported as 1.17 with a 95% confidence interval of (1.02, 1.32). Because this interval does not include the value 1, corresponding to equal proportions in the two groups, we conclude that the higher CVD rates are not the same for the two diets (). The high-salt diet is associated with a 17% higher rate of CVD than the low-salt diet. Note that the relative risk for the individual study in this example was not statistically significant, even though it was higher than the overall estimate (1.19 versus 1.17). This illustrates the value of the meta analysis where the conclusion is based on combining results from several studies.