On the completeness of the class of fixed size sampling strategies

A sampling strategy consists of a sampling design and an estimator. Of special importance are strategies combining simple random sampling with the sample mean as an estimator. We consider this class of strategies and the smaller class of strategies with simple random sampling of (almost) fixed sample size. The convexity of the underlying loss function does not imply the completeness of the smaller class. However the strict convexity of the loss function together with the convexity of its second derivative, is sufficient for this completeness.