Wagenmakers, et al. (in press) claimed that the use of the diffusion model in experimental psychology has been less than widespread because of difficulty in fitting the model to data. They proposed a new method for fitting the model ("EZ") that is simpler than the standard chi-square method. Wagenmakers et al. also suggested that the EZ method can produce accurate parameter estimates in cases where the chi-square method would fail, specifically experimental conditions with small numbers of observations and conditions with accuracy near ceiling. I present a number of comparisons between the two methods: 1. Unlike the chi-square method, the EZ method is extremely sensitive to outlier RTs. 2. It is consistently less efficient in recovering most parameter values. 3. It produces estimates of parameter values that are highly variable (more variable than the chi-square method when the number of observations in a condition is small). 4. Small misspecifications can lead to errors in data interpretation. 5. The proposed tests for misspecification are not powerful enough when the number of observations in an experimental condition is small. I also present a comparison between EZ parameter estimates and chi-square estimates for a published experiment (Ratcliff, Thapar & McKoon, 2003). My conclusion is that the EZ method could be quite useful in exploration of parameter spaces, but should not be used when meaningful estimates of parameter values are needed. |