Solution
= Solution
Replace the expected total <person-time> and failure count in the preceding identity by their observed values, $E=\sum_i x_i$ and $d$. The resulting estimating equation is $\theta E=d$, so
$$
\boxed{\widetilde\theta=\frac{d}{\sum_i x_i}.}
$$
This <events divided by exposure estimator> is failures per unit observed <person-time>. We assume positive total exposure; if $d=0$, it gives the boundary estimate zero.