Observer error and birthweight: digit preference in recording.

We investigated the presence of digit preference in the recording of birthweight by a retrospective analysis of an official dataset of birth registrations. All births occurring in the Province of Saskatchewan in Canada from 1981 to 1991, but excluding 1988, were considered for this analysis. Preference for the terminal digit 0 increased progressively with increasing birthweight over the whole range of birthweights. Correction for digit preference led to an increase of 1.8% in the number of low birthweight babies and a concomitant increase of 0.1% in the actual incidence of low birthweight. Epidemiological applications of birthweight should take into account the observer error caused by digit preference.