Digit preference in estimated gestational age.

A digit preference model is developed to describe the preference for estimating gestational age at birth as an even week during the 20th to 36th week of gestation. The model incorporates a probability of misclassification to adjacent even weeks at odd gestational ages, while even gestational ages are assumed correctly classified. The model is extended to allow the misclassification probabilities to decrease linearly with week during the period. A piecewise exponential model is used to model relative risks of delivery associated with a previous spontaneous abortion and a model incorporating digit preference is fitted as a generalized bilinear model in GLIM. The estimates of relative risk in the underlying survival model are virtually the same whether the misclassification is incorporated in the model or ignored.