In probability sampling, the sampling fraction is the (known) probability with which cases in the population are selected into the sample.

EXAMPLE: If you were to take a simple random sample with a sampling fraction of 1/10,000 from a population of 1,000,000 cases, each case would have a 1/10,000 probability of being selected into the sample, which will consist of approximately 1/10,000 * 1,000,000 = 100 observations.