A comparison of methods for constructing confidence intervals on contrasts in a three-factor mixed model

This paper compares several methods for constructing a confidence interval on contrasts of fixed effects in a balanced three-factor mixed factorial design with one fixed effect and two random effects. In particular, confidence intervals constructed using PROC MIXED of SAS are compared to other intervals that have been proposed in the literature. Computer simulation is used to compare interval lengths, and determine each method's ability to maintain the stated confidence coefficient.