Look out for intervention effects that are described as average differences.
Sometimes in intervention comparisons an outcome, such as yield, is measured on a scale. When scales are used, studies usually report the average difference between the outcome in one intervention group and in the comparison group. But the average effect is not the same for all samples in the group. For example, in some cases there may be a big increase in yield, and in others there may be no increase in yield.
REMEMBER: Don’t be tricked by average effects. When outcomes are measured using scales, don’t assume that the average effect occurs in all situations.