Discretizing continuous variables in nutrition and obesity research: a practice that needs to be cut short.

BACKGROUND/OBJECTIVES: Nutrition and obesity researchers often dichotomize or discretize continuous independent variables to conduct an analysis of variance to examine group differences. We describe consequences associated with dichotomizing and discretizing continuous variables using two cross-sectional studies related to nutrition. SUBJECTS/METHODS: Study 1 investigated the effects of health literacy and nutrition knowledge on nutrition label accuracy (n = 612). Study 2 investigated the effects of cognitive restraint and BMI on fruit and vegetable (F/V) intake (n = 586). We compare analytic approaches where continuous independent variables were either discretized/dichotomized or analyzed as continuous variables. RESULTS: In Study 1, dichotomization of health literacy and nutrition knowledge for 2 × 2 ANOVA revealed health literacy had an effect on nutrition label accuracy. Nutrition knowledge has an effect on nutrition label accuracy, but the health literacy by nutrition knowledge interaction was not significant. When analyzed using regression, the nutrition knowledge effect was significant. The simple effect of health literacy was also significant when health literacy equals zero. Finally, the quadratic effect of health literacy was negative and significant. In Study 2, dichotomization and discretization of cognitive restraint and BMI were used for three ANOVAs, which discretized BMI in three ways. For all ANOVAs, the BMI main effect for predicting fruit and vegetable intake was significant, the interaction between BMI and cognitive restraint was non-significant, and cognitive restraint was only significant when both variables were dichotomized. When analyzed using regression, the continuous mean-centered variables, and their interaction each significantly predicted F/V intake. CONCLUSIONS: Dichotomizing continuous independent variables resulted in distortions of effect sizes across studies, an inability to assess the quadratic effect of health literacy, and an inability to detect the moderating effect of BMI. We discourage researchers from dichotomizing and discretizing continuous independent variables and instead use multiple regression to examine relationships between continuous independent and dependent variables.

Impact of Veteran Status and Timing of PTSD Diagnosis on Criminal Justice Outcomes.

Previous research has demonstrated that jurors show a bias towards treatment for veterans with post-traumatic stress disorder (PTSD). The present research examines this bias when jurors are faced with cases of potential malingering, in which the defendant's claim of PTSD is a perceived attempt to escape legal punishments. Trial vignettes, in which veteran status and PTSD diagnosis timing were manipulated, were used to explore this phenomenon. It was found that veterans who received their diagnosis after being arrested were found guilty more often, and were diverted to treatment less often, than those who were diagnosed before an arrest. This has critical implications for mental healthcare in that it is crucial to properly diagnose and treat people before they find themselves in court. Further, the negative outcomes in court demonstrate one of the severe social impacts of untreated or late-diagnosed PTSD.