Epidemiology of PCBs and neurodevelopment: Systematic assessment of multiplicity and completeness of reporting

Abstract Background Assessments of scientific evidence often involve a systematic comparison of findings within and across studies. An important consideration in systematic reviews is multiplicity, which can arise from simultaneous consideration of multiple independent and dependent variables, use of multiple statistical models, or multiple subgroup analyses. Multiplicity can affect the results of evidence assessment, if accompanied by incomplete or selective reporting. Birth cohort studies investigating prenatal/neonatal exposure to polychlorinated biphenyls (PCBs) and their relation to neurodevelopmental measures during follow up offer an interesting opportunity for assessing multiplicity and completeness of reporting because the literature on the subject is voluminous, and the data typically allow considerable flexibility in terms of choice of analysis and reporting of findings. Methods Following a systematic search, each relevant publication was characterized with respect to its methods of exposure assessment, outcome characterization, analysis and reporting. Based on the total number of unique exposure and outcome categories we calculated the number of possible exposure-outcome associations that could have been examined within and across cohorts. Each association was categorized as “reported” or “not reported/not evaluated”, and the number of studies that reported each association was ascertained. Results A total of 208 prenatal/neonatal exposures and 461 outcomes were measured across 34 cohorts and 111 publications. Only 29 associations were presented in at least three studies; of those, only 21 associations were reported within the same age group allowing a meaningful side-by-side comparison. Percentage of within-cohort associations that were reported among all those that could have been reported based on the available data ranged from 6% to 100%. Conclusions The literature on PCBs and neurodevelopment exemplifies a situation wherein despite large numbers of published studies, inconsistent or incomplete reporting of multiple results impede systematic reviews.