Last time we talked about the unique contributions of a systematic review. Today we will talk about how meta-analysis informs our practice. Remember, these are both techniques that synthesize existing data. This means just like a systematic review, a meta-analysis must be performed with rigor. A very specific question should be asked, and inclusion and exclusion criteria defined before collecting available studies. Multiple databases must be searched, and counts of excluded studies and the reasons should be kept.
Where a meta-analysis differs is in the actual analysis. Remember when we talked about the importance of sample size to obtaining accurate estimates? Meta-analysis uses statistical techniques to pool the data from multiple studies providing us with better estimates. Here is an example: Vaginal birth after two caesarean sections (VBAC-2)-a systematic review with meta-analysis of success rate and adverse outcomes of VBAC-2 versus VBAC-1 and repeat (third) caesarean sections.
Notice that in this abstract we are told which databases were searched, what search terms were used and what studies were found? This is the same as providing methods in clinical research because it allows us to repeat the research if we choose so we can verify the findings.
Not only can a meta-analysis provide better understanding by pooling data, it can also use statistical techniques to inform about publication bias. Publication bias happens when researchers (or publishers) are more likely to report positive findings than negative findings. This means research is more likely to report that an intervention worked than that it didn’t. By analyzing the distribution of the pooled data, meta-analysis techniques can identify when publication bias has likely skewed our understanding of a topic.
Like systematic reviews, meta-analyses are powerful tools for helping make sense of the available research. If you want to become more skilled at reading them, consider following this tutorial from Michigan State University on how to read a Meta-Analysis.
We have one more type of study to talk about before we wrap-up this series, and we will discuss that on Monday.