D. G. Horvitz

D. G. Horvitz coauthored the 1952 paper that generalized finite-population sampling without replacement to unequal selection probabilities. The joint work with D. J. Thompson provides the foundation for the Horvitz-Thompson estimator.^[ref]

The central idea is to compensate for unequal chances of inclusion. For fixed population values $y_i$ and positive inclusion probabilities ${\pi }_i$, write a total estimate asUsing an inclusion indicator $I_i$ gives $\mathbb{E}(I_i/{\pi }_i)=1$, so $\mathbb{E}\hat{Y}={\sum }_i{y}_i$. Unbiasedness follows from the design probabilities rather than from assuming that the sampled values are an independent realization of a population model.^[ref]

For an illustrative population $(4,6)$, select exactly one unit with probabilities $(1/4,3/4)$. The total estimate is $16$ or $8$, respectively, with expected value $(1/4)16+(3/4)8=10$, the true total. The less frequently selected unit receives the larger multiplier. This example explains how a sampling design can deliberately favor some units without forcing an unweighted analysis to be correct.