Simultaneous models for contingency tables: A reanalysis of Heatherton et al. (1997) data on eating behavior

In panel studies, where a categorical response is measured at two points in time, we can examine two kind of hypotheses regarding the nature of change . The first is related with change at the individual level (gross change) t hrough the modelling of joint distribution of responses. The second is rela ted with aggregate change (net change) through the modelling of marginal di stributions of responses. This paper describes a general approach to the an alysis of two-wave panel data based on Lang and Agresti's work (1994) that simultaneously permits the modelling of marginal and joint distributions of responses. This approach is illustrated with data from Heatherton et al. ( 1997) about change in dieting behaviour. These data were originally analyze d using the chi (2) statistic to test independence of responses. This paper shows how it is possible to obtain a better understanding of these data us ing the proposed methodological approach.