Abstract #300376

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JSM 2003 Abstract #300376
Activity Number: 299
Type: Contributed
Date/Time: Tuesday, August 5, 2003 : 2:00 PM to 3:50 PM
Sponsor: Biometrics Section
Abstract - #300376
Title: The Effect of Dichotomizing a Polychotomous Data in Interobserver Agreement Studies
Author(s): Mekibib Altaye*+ and Allan P. Donner
Companies: Childrens Hospital Medicial Center and University of Western Ontario
Address: 3333 Burnet Ave., Cincinnati, OH, 45229-3026,
Keywords: interobserver ; sample size ; agreement ; dichotomous ; polychotomous
Abstract:

Some researchers often tend to dichotomize a polychotomous data either to enhance interpretability or to simplify the statistical analysis. However, such an approach can lead to loss of efficiency in terms of sample size. We investigate the effect of such dichotomization in interobserver agreement studies, focusing on its implication on the required sample size. Dirichlet multinomial distribution was used to model the joint distribution of the polychotomous outcome variable. Based on this model we developed a one degree of freedom goodness-of-fit test that can be used to facilitate sample-size requirements using standard theory of the noncentral chi-square distribution. We then investigate the effect of dichotomizing a polychotomous outcome on sample size, by combining adjacent categories. A prespecified power and precision dichotomization always lead to a substantial increase in the required sample size. However, some of these losses may be recovered by increasing the number of raters, although there seems to be a diminishing return associated with that.

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