# Interpretation of time-to-event curves

## Time-to-event curves

This topic is covered by the LITFL entry, Hazard Ratio, Median Ratio and Kaplan-Meier Curves. It refers to curves of events over time, eg. survival over time.

### Hazard ratio

• Hazard rate = rate at which an event happens over time
• Hazard ratio = treatment hazard rate / control hazard rate
• Interpreted along a measure of time, eg. over 1 year
• "odds of winning the race" - not the margin of victory
• HR of 1 = equal event rate between treatment and control groups
• HR of 2 = twice the number of patients will have the event in the active treatment group
• HR of 0.5 = half the number of patients will have the event in the active treatment group

### Median ratio

• "the margin of victory" - not odds of winning the race.
• Median time = time at which 50% of cases "win the race"
• Contrast with Mean time which is the average resolution rate for that group
• This measures how much faster the treatment achieves an outcome when compared to placebo)
• Median ratio = placebo median time / treatment median time

### Kaplan-Meier curve

• Length of time from study entry to disease end-point
• Compares treatment group and control group
• Median time and mean time are derived from these curves