Empirical and Gardosi model based standard deviation of birthweight at different levels of mean birthweight.
fits
REGRESSION
The constant coefficient variation assumption in Gardosi model would imply the empirical standard deviations would fall along a straight line (red line) with constant CV value as its slope. However, the empirical based fitted line (blue line) differs from the line based on the Gardosi assumption and the empirical points don’t show a linear pattern. Data are from the NICHD Fetal Growth Studies–Singleton. The sample size of each group is 5, 25, 85, 292, 668, 822, 350, and 41 for the point representing the grouped bins from smallest to largest predicted birthweight, respectively.We further investigated the assumption of normality by checking the residuals from the model fits for the Gardosi and the heteroscedastic models. The quantile regression does not assume normality and hence it was not included in the investigation. The residuals did not show any glaring departure from normality (