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Estimation, Evaluation, and Selection of Actuarial Models

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2.2. ESTIMATION USING DATA-DEPENDENT DISTRIBUTIONS 17<br />

Integrating both sides yields<br />

Z t Z<br />

ds(u) t<br />

0 r(u) = h(t)dt = H(t).<br />

0<br />

Now replace the true expected count s(t) by ŝ(t), the observed number <strong>of</strong> deaths by time t. Itisa<br />

step function, increasing by s i at each death time. Therefore, the left-h<strong>and</strong> side becomes<br />

X s i<br />

r i<br />

which defines the estimator, Ĥ(t). The Nelson-Åalen estimator is<br />

⎧<br />

⎪⎨ 0, 0 ≤ t

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