Errors in deformation-density and valence-density maps: the scale-factor contribution

A formulation for the variance-covariance estimate of the deformation or valence density is given for a centrosymmetric crystal, taking into account correlations with the scale factor, when this is determined by the usual scaling procedure. It is shown that, when heavy atoms are present in the crystal, the scaling procedure is likely to result in better precision than an independent experimental determination of the scale factor.