The curves shown in Figure 6 are known as Kaplan Meier (KM) plots or survival curves (although they can be plotted for time to any event, not just death). The proportion of patients surviving is plotted on the yaxis against the time after randomisation on the xaxis. Two curves are drawn, one for the treatment A group and the other for the treatment B group. Under the xaxis are the number of patients who are at risk (have not reached the endpoint) at the times shown and, in brackets, the number of events between these times. The data are not taken from a real trial because of issues such as confidentiality.
The short vertical lines on each curve denote censored patients. When a patient is censored, their followup only goes up to this point, and they have not yet reported the event. Although we know they are still alive they no longer count as a ‘patient at risk’ after this time on the graph. Therefore, they contribute no information further to the right of the curve. In the calculations the effect of censoring is to reduce the ‘number at risk’ without affecting the shape of the survival curve.
Censored patients are the reason why the data in brackets under the xaxis are not straightforward sums e.g. on Arm A the number of patients at risk at 24 months (174 patients) is not the number of patients at risk at 12 months (272 patients) minus the events on between 12 and 24 months (52 patients) i.e. 46 patients have been censored between these points.
Statisticians use datasets like the one presented here to calculate the Hazard ratio HR. The hazard ratio is the ratio of the hazards or risks for patients on the two treatments. The hazard relates to the endpoint of interest e.g. the chance or risk of death or progression of the disease or fall in CD4 cells. A calculation using all the data in Figure 5 gives a Hazard Ratio of 0.66. This represents a 34% reduction in the risk of death on arm A. (Note that HR = 1.0 would represent no difference between the treatment arms.)
