Evaluation of a meta-analysis of air quality and heart attacks, a case study

TL;DR

This study critically evaluates a meta-analysis linking air quality to heart attacks, revealing potential biases and manipulation in the statistical evidence, and questions the reliability of the reported associations.

Contribution

It introduces a method to assess the reliability of base studies in meta-analyses using p-value plots and analysis search space evaluation.

Findings

Evidence of p-value manipulation in base studies

Large analysis search spaces suggest potential bias

Meta-analysis findings are unreliable due to biased p-values

Abstract

It is generally acknowledged that claims from observational studies often fail to replicate. An exploratory study was undertaken to assess the reliability of base studies used in meta-analysis of short-term air quality-myocardial infarction risk and to judge the reliability of statistical evidence from meta-analysis that uses data from observational studies. A highly cited meta-analysis paper examining whether short-term air quality exposure triggers myocardial infarction was evaluated as a case study. The paper considered six air quality components - carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter 10 and 2.5 micrometers in diameter (PM10 and PM2.5), and ozone. The number of possible questions and statistical models at issue in each of 34 base papers used were estimated and p-value plots for each of the air components were constructed to evaluate the effect heterogeneity of p-values used from the base papers. Analysis search spaces (number of statistical tests possible) in the base papers were large, median of 12,288, interquartile range: 2,496 to 58,368, in comparison to actual statistical test results presented. Statistical test results taken from the base papers may not provide unbiased measures of effect for meta-analysis. Shapes of p-value plots for the six air components were consistent with the possibility of analysis manipulation to obtain small p-values in several base papers. Results suggest the appearance of heterogeneous, researcher-generated p-values used in the meta-analysis rather than unbiased evidence of real effects for air quality. We conclude that this meta-analysis does not provide reliable evidence for an association of air quality components with myocardial risk.