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

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

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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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