Rounding of birth weights in a neonatal intensive care unit over 20 years: an analysis of a large cohort study

To determine the frequency of birth weight digit preference for infants admitted to a large neonatal intensive care unit (NICU), the scale of rounding and its dependence on birth weight, and time and the impact on prescribing accuracy.

A consecutive cohort of birth weights extracted retrospectively from a single clinical database.

Birth weights from 9170 inborn infants recorded on an electronic prescribing database admitted to NICU over 20 years.

Data are presented for the frequency of each of the possible pairs of final digits. A statistical model of digit preference assuming rounding is used to quantify the proportions rounding to specific accuracy levels. These proportions are compared between those <1000 g and those above and over the 20-year time period.

From a population of 9170 infants admitted over 20 years, there was a highly statistically significant digit bias with an increased prevalence of multiples of 100 (p<0.0001), 50 (p=0.007), 20 (p<0.0001), 10 (p<0.0001), 5 (p<0.0001) and 2 (p=0.0005). There was clear evidence of a reduced 100 g digit bias for infants 500 and 1000 g (0%) compared with those between 1000 and 4500 g (3.7%). The maximum birth weight error due to digit bias for all infants was 5%. There was clear evidence of an improvement in accuracy over 20 years.

Digit bias in birth weights over 20 years in a tertiary NICU is highly significant at the 100, 50, 20, 10, 5 and 2-digit levels. There has been a substantial improvement in the accuracy of birth weight measurements over 20 years. The likely maximum error due to birth weight digit bias is 5% and is within an acceptable tolerance for drug dosing even at very low birth weights.