The previous two statistical questions described survival (time to event) data.[1, 2] The example used was a randomised controlled trial that evaluated the effectiveness of an integrated care programme compared with usual care in facilitating the return to work of patients with chronic low back pain.[3] The integrated care programme was a combined patient and workplace directed intervention.
Trial participants were adults aged between 18 and 65 years who had experienced low back pain for more than 12 weeks, were in paid work, and were absent or partially absent from work. The primary outcome was duration of time off work—that is, from randomisation until a fully sustained return to work.
Participants were followed for 12 months. The survival (time to event) data for the two treatment groups were compared statistically using the log rank test (P=0.003). Days until a fully sustainable return to work for the group on the integrated care programme and the usual care group were plotted as Kaplan-Meier survival curves.
Figure 1
Kaplan-Meier survival curves showing the time until a fully sustained return to work for patients in the integrated care programme and those receiving usual care. Censored observations are displayed by crosses on the curves
Which of the following statements, if any, are true?
a) The log rank test facilitates testing of the null hypothesis—in this case, that there are no differences in survival times in the population between the integrated care programme and usual care
b) Censored observations were excluded before the interventions were compared statistically
c) A statistically significant difference in survival times existed between interventions at the 5% level of significance
d) The log rank test provides an estimate of the magnitude of the difference in survival times between interventions
Answers
Answers a and c are true; b and d are false.