Correlation of marker values in Down's syndrome screening: the effect of dating error

Central to any screening algorithm for Down's syndrome are the values used for the parameters in the multivariate Gaussian statistical model that is used to describe the joint distribution of the marker values. There has been much discussion about the values of the means and standard deviations which are appropriate but little interest has been shown in the values of the correlation coefficients between markers. There has been some speculation that the range of parameter values quoted in the literature arises from factors such as storage of samples, between-assay effects and differences in assay methodologies. We show that gestational dating error, among other factors, could be responsible for much of the variation that is present in quoted parameter values, though the factors mentioned above clearly have an effect.