Figure 1.

Figure 1. From: Pooling Dietary Data Using Questionnaires With Open-ended and Predefined Responses: Implications for Comparing Mean Intake or Estimating Odds Ratios.

Graphic representation of predefined categories versus open-ended responses regarding frequency of intake showing possible noncontiguous categories and the gaps (G) between the categories. A) The underlying exponential distribution of intake; each color denotes a predefined category and white spaces denote a gap. The gaps are left by the original categories, especially when translated into times per year from the original categories. Individuals in the gaps were categorized as shown in B) (bars show where the cutpoints for the gaps fall on the original distribution, with the colors showing which categories the individuals in the gaps were categorized into for each of the following 7 methods of categorizing: 1) Midpoint: the midpoint of the gap was used as a cutpoint; individuals who fell below this point were assigned to the previous category. If they fell above this midpoint, they were assigned the next category. 2) Median: the median of the individuals in the gap was used as the cutpoint. Above the value was assigned to the previous category and below the value to the next category. 3) Quartile 1: The first quartile of the individuals in the gaps was used as the cutpoint to determine to which category an individual belonged. 4) Quartile 3: The third quartile of the individuals in the gaps was used as the cutpoint to determine to which category an individual belonged. 5) Minimum: everyone in a gap was assigned to the next category (the minimum of the gap is the cutpoint). 6) Maximum: everyone in a gap was put in the previous category (the maximum of the gap is the cutpoint). 7) Everyone in the gap was treated as missing, represented by the white space.

Michael D. Swartz, et al. Am J Epidemiol. 2010 March 15;171(6):682-690.

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