Using a health-services study as an illustrative example of longitudinal randomized field research with the potential for participants to be lost to follow-up, the researchers applied a permutation test where the treatment indicator variable is randomly permuted in the context of regression models with covariates and attrition weighting. 

The test is applied to a multi-site randomized intervention trial of a quality-improvement program for adolescent depression treatment in primary-care settings, in which regression models were used to assess intervention effects with weights used to adjust for attrition bias.