Biostatistics in Research Practice: The ESPRIT Trial, suggested answer 3

Two graphs side by side, entitled 'Primary outcome event' and 'Ischaemic events'. Each has 'Time from randomisation (years)' on the horizontal axis and 'Cumulative event rate (%)' on the vertical axis. Each graph shows two rising stepped lines labeled 'Aspirin alone' and 'Aspirin + dipyridamole'. The aspirin alone line rises more steeply in each graph.

3. What type of graph is this? Why is the vertical axis labelled “Cumulative event rate”?

Suggested answer

This is a Kaplan Meier survival curve, showing the proportion who have experienced the event rather than the proportion who have not yet experienced the event, i.e. the proportion surviving.

It is calculated as follows. At each time when there is an event, we calculate the proportion who do not experience an event. We multiply these together for all the times up to the index time to give the proportion who have not experienced the event up to and including this time. For this particular curve, they have subtracted this proportion from one to give the proportion who have experienced the event so far.

It is labelled “Cumulative event rate” because the graph shows the proportion of subjects who have experienced the event by the time shown on the horizontal axis, including all those events which occurred before that time.

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