Introduction Systemic reviews synthesize previous studies to produce more precise and generalizable data synthesized studies can be interventional or observational Meta-analysis is the statistical procedure for combining numerical results from synthesized studies Systemic Review Systematic and procedural method of collecting, evaluating, and synthesizing qualitative or quantitative literature to answer a question methods and selection criteria must be specified in advance May or may not include meta-analysis Meta-Analysis Statistical method to combine data from multiple studies weighted average Advantages better precision than individual studies improves generalizability of study findings considered to be the highest level of clinical evidence Limitations quality of individual studies bias of individual studies must be assessed and accounted for variability in study methods studies must be similar enough to be meaningfully combined e.g., meta-analysis may not be helpful for a study looking to compile evidence about the effects of reading on depression scores if the primary studies all focused on reading different genres would be more helpful to answer a specific question such as "How does reading fiction influence depression scores?" study heterogeneity when variation in effect size of included studies is greater than would be expected by pure chance e.g. one study demonstrates a massive treatment effect and another demonstrates a negligible one could be due to differences in participant characteristics treatment could have larger impact in certain populations and pooling the results without examining that possibility will obfuscate that statistical concerns high heterogeneity could suggest absence of "true" effect, rendering pooled data meaningless subject to publication bias outcome and statistical significance of a study or experiment influences the likelihood of its publication more papers with positive results are published despite similar quality when present, sampled publications are not a true representation of gathered evidence may lead to increase in false conclusions Funnel Plot Evaluates for the presence of publication bias Used in systemic reviews and meta-analyses Plots precision versus results measure (e.g. odds ratio, mean, and relative risk) of included studies Should take on a funnel (triangle) shape studies with high precision (high value on y-axis) should fall near the average result measure (center of x-axis) studies with low precision should fall scattered on either side of the average result Deviation from funnel shape indicates publication bias Forest Plot Used in systemic reviews and meta-analyses to visually display results from individual studies Vertical "line of null effect" at point on horizontal axis representing no association between exposure and outcome Positive and negative results measures (e.g., odds ratio, mean, and relative risk) of included studies plotted as boxes or shapes on either side of the line relative size of the shape indicates relative size of the study 95% confidence interval (CI) plotted as a line extending horizontally from each point any CI crossing the vertical line indicates the results are statistically insignificant Combined point estimate (vertical line) and confidence intervals represented by a diamond