Meta-analyses are supposed to combine the evidence on a topic from many studies (e.g., does melatonin help sleep?) to produce an answer. Many people even consider them the gold standard for evidence about scientific questions. Unfortunately, taking a weighted average of many different studies sometimes doesn't work because averaging the studies can be meaningless. Suppose a meta-analysis on "meditation for depression" tries to average the results of a one-hour app-based mindfulness medita...