Most of us take a qualitative approach to reviewing the literature for no reason other than that’s what we were taught to do or what we’ve read. But there are at least four problems with narrative or literature reviews:
- they are rarely comprehensive
- they are highly susceptible to reviewer bias
- they seldom take into account differences in the quality of studies
- they often come to the wrong conclusion or no conclusion at all, hence the oft-heard call for further research
At best, a literature review may be able to inform a conclusion about the direction of an effect. But a meta-analysis will provide you with a point estimate of the effect size and a confidence interval quantifying the precision of the estimate.
Meta-analysis will normally permit you to reach a conclusion even when the underlying data come from dissimilar studies reporting conflicting conclusions.