Fixed cost sequential sampling schemes are introduced in this article. In these schemes units are observed sequentially according to a given sampling method until the total cost reaches a preassigned value; it is assumed that the cost of examining each unit is unknown in advance. It is shown how the notion of sufficiency in sampling can be used to construct unbiased estimators of population parameters under these schemes.
"Unbiased Estimation in Fixed Cost Sequential Sampling Schemes." Ann. Statist. 4 (5) 1012 - 1017, September, 1976. https://doi.org/10.1214/aos/1176343601