In sampling theory

Sampling theory

sampling theory may mean:* Nyquist–Shannon sampling theorem, digital signal processing * Statistical sampling* Fourier sampling...

, sampling fraction is the ratio of sample size to population size or, in the context of stratified sampling

Stratified sampling

In statistics, stratified sampling is a method of sampling from a population.In statistical surveys, when subpopulations within an overall population vary, it is advantageous to sample each subpopulation independently. Stratification is the process of dividing members of the population into...

, the ratio of the sample size to the size of the stratum.
The formula for the sampling fraction is = n/N, where n is the sample size and N is the population size. If the sampling fraction is less than 5%, then the finite population effect might be ignored.