Understand the relationship
The reasoning behind the result
A fixed cohort defines the question
Survivorship at a follow-up point asks what fraction of the originally defined cohort remains alive. New entrants belong to another cohort; simply comparing two open-population counts cannot answer this question.
The record must distinguish confirmed outcomes from unknown follow-up. The tool does not infer death from absence, disappearance, failed detection or a missed visit.
Unknown outcomes are not ordinary censoring analysis
This calculator shows a known-status fraction and logical full-cohort bounds. It does not implement Kaplan–Meier estimation or use individual censoring times. Calling the unknown outcomes 'censored' does not supply the timing information those models need.
If you have complete repeated counts of a closed cohort, the related life-table tool can describe changes across the recorded intervals. If individuals enter, leave or are lost at different times, use an analysis appropriate to that design.
Keep observed fractions distinct from forecasts
The confirmed survivor count is a measurement record. A percentage formed from that count does not predict an individual's lifespan or establish future survival probability.
Pooling records weights by their assessed counts. It does not account for exposure duration, treatment assignment, population differences or dependence among observations.
Follow the numbers
Unequal cohorts pooled by counts
- Cohort A records 80 survivors among 100; cohort B records 15 among 25.
- Together they record 95 survivors among 125 fully assessed individuals.
- Pooled survivorship = 95/125 × 100 = 76%, rather than the equal-cohort average of 70%.
The pooled fraction weights individuals; the individual cohort rows remain necessary to understand its composition.
Quick guide
How to use this calculator
- Enter the assessment period and each group's individuals in the initial cohort.
- Record confirmed confirmed survivors separately from unknown outcomes.
- Read the assessed denominator and the full-cohort bounds before comparing percentages.
- Pool only cohorts whose observation period and outcome definition make a combined fraction meaningful.
Calculation method
Calculation and interpretation
Describe the observed survivorship of an original biological cohort at one stated follow-up point.
Assessed = initial − unknown; observed fraction = confirmed/assessed; whole-cohort bounds = confirmed/initial to (confirmed + unknown)/initial
Worked example
Unequal cohorts pooled by counts
The pooled fraction weights individuals; the individual cohort rows remain necessary to understand its composition.
Assessed = initial − unknown; observed fraction = confirmed/assessed; whole-cohort bounds = confirmed/initial to (confirmed + unknown)/initial
Supported inputs
Precision and limits
Descriptive cohort records
A recorded fraction does not establish a biological cause or predict an individual's outcome. Keep counting definitions, observation period and missing-status rules consistent. No treatment, husbandry or management action is prescribed.
Unknown outcomes are not failures
The assessed fraction excludes unknown status. The full-cohort bounds show the possible fraction if all unknowns were non-events or all were events; they are logical bounds, not a statistical confidence interval.
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