Fig. 1.

Different sources of variability may contribute to the diffusion data of control subjects (CO) and patients with Huntington's disease (HD). Although the contribution of each source of variation cannot be singled out, the coefficient of variance (CV) from the CO data of each center serves as an estimate whether one or more centers contributed systematically biased data. Investigating FA group differences between patients and controls on systematic between-center differences is a further criterion to decide whether pooling across centers is permitted. The calibration procedure as well as the inclusion of more subjects is an option and is an alternative if the various checks do not permit pooling the data.