Digit preference bias in the recording of emergency department times.

OBJECTIVE Digit preference bias has previously been described in a number of different clinical settings. The paper aimed to assess whether digit preference bias affects the recording of the time patients arrive and leave emergency departments. METHOD An observational study of 137 emergency departments in England and Wales was conducted. Each department was asked to submit details of the time of arrival and time of departure from the emergency department for each patient attending during April 2004. In addition, interviews with the lead clinician were undertaken to determine the method used to record the time of departure. The degree of digit preference bias was assessed using a modification of Whipple's index. RESULTS One hundred and twenty-three (86.9%) departments submitted data detailing 648,203 emergency department episodes. 114,875 (18.0%) episodes had a recorded minute of departure of '0' or '30', with a further 281,890 (44.1%) having other values with a terminal digit of '0' or '5'. The mean modified Whipple's index for time of departure was 316.9 (range 70.9-484.4). Linear regression demonstrates a small but significant inverse relationship between the modified Whipple's index and the mean total time in department (b = -0.05, 95% CIs -0.09 to -0.0004, P = 0.048). CONCLUSION Some departments show considerable digit preference bias in the recording of time of departure from the emergency department. Such bias may cause difficulty in assessing changes in the performance of departments.