Look out for treatment effects that are described as average differences.
Sometimes in treatment comparisons, an outcome, e.g. fluency of speech, is measured on a rating scale. When scales are used, studies usually report the average difference between the outcome in one treatment group and in the comparison group. But not everyone experiences the average effect. For example, some people may experience a big increases in fluency following a treatment and others may not experience a reduction in dysfluency.
REMEMBER: Don’t be tricked by average effects. When outcomes are measured using scales, don’t assume that everyone experiences the average effect.