Systematic reviews and meta-analyses are a distinct type of empirical studies – they use other, original empirical studies as their "sample," to summarize their findings (i.e., evidence) related to a particular topic or intervention. The idea behind a systematic review is to make sure that an analysis of empirical literature on a specific topic is as comprehensive and unbiased as possible. Meta-analyses go a step further: besides including all relevant studies on a specific topic, researchers summarize the key results not just in a narrative fashion but also by calculating an average size of the relationship between two variables as a numerical result across all studies included in the meta-analysis. In a meta-analysis, researchers would calculate the average difference in outcomes between the treatment and control groups, to help us understand not only how effective a specific intervention is but also how much more effective it is than the alternative approach.