Before revision:  combining data from different scanners and sites for structural covariance analysis in a naive manner should be avoided,  (2) surface area and volume should be preferred as morphological measures of structural covariance over cortical thickness , and (3) some analysis of robustness of the results should be performed for every structural covariancestudy .
After revision:  combining data from different scanners and sites for structural covariance analysis in a naive manner should be avoided, (2) surface area and volume should be preferred as morphological measures of structural covariance over cortical thickness , and (3) a large number of subjects should be used to estimate structural covariance; (4) measurement error should be assessed where repeated measurements are available; (5) if combining sites is critical, univariate site-correction is insufficient, but error covariance should be explicitly measured and modelled .