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Survival Analysis · MEDSURV-CI

Sample Size for Estimating Median Survival Time Using the Kaplan–Meier Method

Plan an approximate confidence-interval margin for a single Kaplan–Meier median survival estimate.

1. Planning setup

Use the selected unit consistently for all time inputs.

2. Information obtained from previous study / literature

A + F is maximum follow-up for the earliest participant; F is minimum follow-up for the last participant.

3. Information decided by the researcher

4. Results

Enter assumptions and select Calculate.

Formula, assumptions, limitations, and methodological note

Example clinical scenario

A registry study wants to estimate median overall survival after diagnosis of advanced lung cancer with a confidence interval of specified precision.

Formula and methodological note

This is an asymptotic Kaplan–Meier variance and delta-method planning approximation, not an exact finite-sample Brookmeyer–Crowley interval-width calculation. Exponential survival is used only for planning.

Absolute margin of error specifies a fixed time-scale precision target. For example, an anticipated median of 12 months with a margin of 3 months targets an approximate interval of 12 ± 3 months.

Relative precision expresses the margin as a percentage of the anticipated median. For example, 25% of an anticipated 12-month median is a derived absolute margin of 3 months. The two modes are equivalent when the absolute margin equals the relative precision multiplied by the anticipated median.

Use absolute precision when a clinically meaningful number of days, weeks, months, or years is the relevant target. Use relative precision when proportional accuracy is more appropriate across different anticipated median survival times.

Method references

  1. Brookmeyer R, Crowley J. A confidence interval for the median survival time. Biometrics. 1982;38(1):29–41.
  2. Kaplan EL, Meier P. Nonparametric estimation from incomplete observations. J Am Stat Assoc. 1958;53:457–481.
  3. Greenwood M. A Report on the Natural Duration of Cancer. London: HMSO; 1926.

Suggested citation

Yaacob, Najib Majdi. “Sample Size for Estimating Median Survival Time Using the Kaplan–Meier Method (MEDSURV-CI).” Web calculator. NajibMY, 2026. Accessed July 29, 2026. https://najibmy.netlify.app/tools/sample-size/survival-analysis/estimating-median-survival-time/.