Evaluation of a meta-analysis of ambient air quality as a risk factor for asthma exacerbation
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False-positive results and bias may be common features of the biomedical literature today, including risk factor-chronic disease research. A study was undertaken to assess the reliability of base studies used in a meta-analysis examining whether carbon monoxide, particulate matter 10 and 2.5 micro molar, sulfur dioxide, nitrogen dioxide and ozone are risk factors for asthma exacerbation (hospital admission and emergency room visits for asthma attack). The number of statistical tests and models were counted in 17 randomly selected base papers from 87 used in the meta-analysis. P-value plots for each air component were constructed to evaluate the effect heterogeneity of p-values used from all 87 base papers The number of statistical tests possible in the 17 selected base papers was large, median=15,360 (interquartile range=1,536 to 40,960), in comparison to results presented. Each p-value plot showed a two-component mixture with small p-values less than .001 while other p-values appeared random (p-values greater than .05). Given potentially large numbers of statistical tests conducted in the 17 selected base papers, p-hacking cannot be ruled out as explanations for small p-values. Our interpretation of the meta-analysis is that the random p-values indicating null associations are more plausible and that the meta-analysis will not likely replicate in the absence of bias. We conclude the meta-analysis and base papers used are unreliable and do not offer evidence of value to inform public health practitioners about air quality as a risk factor for asthma exacerbation. The following areas are crucial for enabling improvements in risk factor chronic disease observational studies at the funding agency and journal level: preregistration, changes in funding agency and journal editor (and reviewer) practices, open sharing of data and facilitation of reproducibility research.
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