Abstract: This paper is motivated by an investigation into the growth of pigs, which studied among other things the effect of short–term feed with drawal on live weight. This treatment was thought to reduce the variability in the weights of the pigs. We represent this reduction as an attenuation in an animal–specific random effect. Given data on each pig before and after treatment, we consider the problems of testing for a treatment effect and measuring the strength of the effect, if significant. These problems are related to those of testing the homogeneity of correlated variances, and re gression with errors in variables. We compare three different estimates of the attenuation factor using data on the live weights of pigs, and by simulation.