TheNelson–Aalen estimator is anon-parametric estimator of thecumulative hazard rate function in case ofcensored data orincomplete data.[1] It is used insurvival theory,reliability engineering andlife insurance to estimate the cumulative number of expected events. An "event" can be the failure of a non-repairable component, the death of a human being, or any occurrence for which the experimental unit remains in the "failed" state (e.g., death) from the point at which it changed on. Theestimator is given by
with the number of events at time and the total individuals at risk at.[2]
The curvature of the Nelson–Aalen estimator gives an idea of the hazard rate shape. A concave shape is an indicator forinfant mortality while a convex shape indicateswear out mortality.
It can be used for example when testing the homogeneity ofPoisson processes.[3]
It was constructed byWayne Nelson andOdd Aalen.[4][5][6]The Nelson-Aalen estimator is directly related to theKaplan-Meier estimator and both maximize theempirical likelihood.[7]