A systematic review yields pooled effects
The most important part of conducting systematic reviews constitutes the effort put forth by the researchers to search for all of the relevant literature that exists within accessible databases, paper publications, unpublished studies, and even studies in foreign languages. Going further, once a thorough literature search has been conducted, an even more objective and methodologically sound effort must be made by the researchers to assess the quality, validity, and generalizability of individual study methods and results for inclusion in the systematic review. The inclusion and exclusion criteria for individual studies in a systematic review must be operationally defined in an overt and explicit fashion.
When the findings of several quality RCTs are aggregated in a meta-analysis, a much more precise estimate of the treatment effect can be generated due to increased sample size and representation of the population. Subgroup analyses or "sensitivity analyses" can be conducted with the large aggregated population brought together across studies in a systematic review.
Forest plot and heterogeneity of pooled effect in a systematic review
The I2 statistic is a measure of heterogeneity amongst the studies' findings which may artificially inflate or deflate the pooled effect. As a general criterion, < 25% is low heterogeneity, < 50% is moderate, and < 75% is considered high heterogeneity. With high values of I2, the ability to appropriately interpret the pooled effect decreases. Sensitivity analyses or subgroup analyses should be conducted to account for high levels of heterogeneity.
Forest plots will often look like the diagram below. Notice that RCTs with smaller squares (small sample sizes) have wider confidence intervals and trials with larger squares (larger sample sizes) have small confidence intervals and weigh more on the pooled effect. The diamond is the pooled effect.
Funnel plots and systematic reviews
The steps for conducting a meta-analysis
2. A systematic review is conducted using explicit inclusion and exclusion criteria across the spectrum of available and relevant sources of evidence.
3. The studies are individually assessed for inclusion into the meta-analysis and are objectively assessed in an independent fashion.
4. The results of the studies that meet inclusion criteria are reviewed and the measures of odds, risk, or treatment effect are recorded.
5. Publication bias is assessed using a funnel plot.
6. Sensitivity analyses are conducted as needed for research questions related to subgroups of the population.
7. The meta-analysis is conducted and the results are reported in a forest plot.
8. The heterogeneity of the effect is assessed using the I2 statistic.
9. If the I2 statistic is acceptable, then the pooled effect is interpreted.
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