Probability Sampling

8.4.2 - Probability Sampling (Rev. 11962; Issued: 04-21-23; Effective: 05-22-23; Implementation: 05-22-23) Regardless of the method of sample selection used, the contractor shall follow a procedure that results in a probability sample. For a procedure to be classified as probability sampling, the following two features must apply: • It must be possible, in principle, to enumerate a set of distinct samples that the procedure is capable of selecting if applied to the target universe. Although only one sample will be selected, each distinct sample of the set has a known probability of selection. It is not necessary to actually carry out the enumeration or calculate the probabilities. All that is required is that one could, in principle, write down the samples, the sampling units contained therein, and the relevant probabilities; and • Each sampling unit in each distinct possible sample must have a known probability of selection. In the case of statistical sampling for overpayment estimation, one of the possible samples is selected by a random process according to which each sampling unit in the target population receives its appropriate chance of selection. The selection probabilities do not have to be equal but they should all be greater than zero. In fact, some designs bring gains in efficiency by not assigning equal probabilities to all of the distinct sampling units. Once a procedure and design that satisfies these above properties has been selected, execution of the probability sampling may occur. If a particular probability sampling design is properly executed, i.e., defining the universe, the sampling frame, the sampling units, using proper randomization, accurately measuring the variables of interest, and using the correct formulas for estimation, then assertions that the sample or that the resulting estimates are “not statistically valid” cannot legitimately be made. In other words, a probability sample and its results are always “valid.” However, because of differences in the choice of a design, the level of available resources, and the method of estimation, some procedures lead to higher precision (smaller confidence intervals) than other methods. A feature of probability sampling is that the level of uncertainty can be incorporated into the estimate of overpayment as is discussed below.