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

Nutrition and obesity researchers often dichotomize or discretize continuous independent variables to conduct an analysis of variance to examine group differences. This paper describes the consequences associated with dichotomizing and discretizing continuous variables using two cross-sectional studies related to nutrition. 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). The authors compare analytic approaches where continuous independent variables were either discretized/dichotomized or analyzed as continuous variables. Dichotomizing continuous independent variables resulted in distortions of effect sizes, an inability to assess the quadratic effect of health literacy, and an inability to detect the moderating effect of BMI. The authors discourage researchers from dichotomizing and discretizing continuous independent variables and instead use multiple regression to examine relationships between continuous independent and dependent variables.

Osvaldo F. Morera, Mosi I. Dane'el, Brandt A. Smith, Alisha H. Redelfs, Sarah L. Ruiz, Kristopher J. Preacher, Leah D. Whigham