Systematic Reviews & Meta-Analysis

A meta-analysis goes beyond a systematic review in the critique and integration of studies, it also conducts secondary statistical analysis on the outcomes of the studies.  A meta-analysis looks to synthesize the results of two or more studies that addressed the same hypothesis in the same way. By combining information from all relevant studies, meta-analysis can provide more precise estimates of the effects found from the interventions or treatments being researched than those derived from the individual studies within a review.

A systematic review and meta-analysis may mistakenly be referred to one another synonymously. This is likely due to many systematic reviews containing a meta-analysis, but a meta-analysis is not necessary to complete a systematic review.  

Additional key differences between the consist of the following:

Systematic Reviews 

• Assesses the validity of the findings of the included studies

• Systematically presents and synthesizes the characteristics and findings of the included studies.

Meta-Analysis 

• Use statistical methods to summarize the results of the included studies.

• Look to gather an overall statistic (together with its confidence interval) that summarizes the effectiveness of an experimental intervention compared with a comparative intervention

Not all topics or studies may be similar enough or have sufficient evidence to allow a meta-analysis to be conducted, which is one reason why systematic reviews are more common than meta-analyses.

Meta-analysis can be conducted when you have a collection of studies that:

• Examine the same concepts and relationships

• Have findings that can be configured in a comparable statistical form (e.g., as effect sizes, correlation coefficients, odds-ratios, etc.)

• Contain “comparable” characteristics, such as:

  • Objective of study (effect or variability)

  • Population of study

  • Type of study (RCT, Case-Control, or Case Report)

Meta-analysis is typically a two-stage process:

1. In the first stage, a summary statistic or standardized index is calculated for each study to describe the effect size of the intervention or treatment being observed. 
2. In the second stage, a summary (combined) intervention effect estimate is calculated as a weighted average of the intervention effects estimated in the individual studies.

There are various methods to conduct a meta-analysis. To find further details, please refer to the resources below:

• Basu, A. (2017). How to conduct meta-analysis: A basic tutorial. PeerJ Preprints 5:e2978v1. 
• Balduzzi, S., Rücker, G., & Schwarzer, G. (2019). How to perform a meta-analysis with R: A practical tutorial. Evidence-based Mental Health, 22(4), 153-160. 
• Chaimani, A., Mavridis, D., & Salanti, G. (2014). A hands-on practical tutorial on performing meta-analysis with stata. Evidence - Based Mental Health, 17(4), 111-116.
• Deeks JJ, Higgins JPT, Altman DG (editors). Chapter 10: Analysing data and undertaking meta-analyses. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.2 (updated February 2021). Cochrane, 2021. Available from www.training.cochrane.org/handbook.
• Russo M. W. (2007). How to Review a Meta-analysis. Gastroenterology & hepatology, 3(8), 637–642.

Greenhalgh T. (1997). Papers that summarise other papers (systematic reviews and meta-analyses). BMJ (Clinical research ed.), 315(7109), 672–675. https://doi.org/10.1136/bmj.315.7109.672

Deeks JJ, Higgins JPT, Altman DG (editors). Chapter 10: Analysing data and undertaking meta-analyses. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.2 (updated February 2021). Cochrane, 2021. Available from www.training.cochrane.org/handbook.