title: A Simulation Study of Categorizing Continuous Exposure Variables Measured with Error in Autism Research: Small Changes with Large Effects.
abstract: Variation in the odds ratio (OR) resulting from selection of cutoffs for categorizing continuous variables is rarely discussed. We present results for the effect of varying cutoffs used to categorize a mismeasured exposure in a simulated population in the context of autism spectrum disorders research. Simulated cohorts were created with three distinct exposure-outcome curves and three measurement error variances for the exposure. ORs were calculated using logistic regression for 61 cutoffs (mean ± 3 standard deviations) used to dichotomize the observed exposure. ORs were calculated for five categories with a wide range for the cutoffs. For each scenario and cutoff, the OR, sensitivity, and specificity were calculated. The three exposure-outcome relationships had distinctly shaped OR (versus cutoff) curves, but increasing measurement error obscured the shape. At extreme cutoffs, there was non-monotonic oscillation in the ORs that cannot be attributed to "small numbers." Exposure misclassification following categorization of the mismeasured exposure was differential, as predicted by theory. Sensitivity was higher among cases and specificity among controls. Cutoffs chosen for categorizing continuous variables can have profound effects on study results. When measurement error is not too great, the shape of the OR curve may provide insight into the true shape of the exposure-disease relationship.
authors: Department of Environmental and Occupational Health, School of Public Health, Drexel University, Philadelphia, PA 19104, USA. karynkh@aol.com.
authors: Karyn
authors: K
authors: Heavner
authors: Department of Environmental and Occupational Health, School of Public Health, Drexel University, Philadelphia, PA 19104, USA. Igor.Burstyn@drexel.edu.|Drexel Autism Institute, School of Public Health, Drexel University, Philadelphia, PA 19104, USA. Igor.Burstyn@drexel.edu.|Department of Epidemiology and Biostatistics, School of Public Health, Drexel University, Philadelphia, PA 19104, USA. Igor.Burstyn@drexel.edu.
authors: Igor
authors: I
authors: Burstyn
mesh_terms: D001321:Autistic Disorder / Q000139:chemically induced / Q000453:epidemiology*; D003198:Computer Simulation; D004781:Environmental Exposure; D004785:Environmental Pollutants / Q000633:toxicity*; D006801:Humans; D016015:Logistic Models; D008137:Longitudinal Studies; D008962:Models, Theoretical; D016017:Odds Ratio; D012107:Research Design; D012680:Sensitivity and Specificity
chemical_list: D004785:Environmental Pollutants
keywords: autism spectrum disorders; categorization; dichotomization; epidemiology methods; misclassification