Different sampling schemes

Two different ways to collect a random sample of n values from a finite population of size N are common. In both sampling schemes, each population value has the same chance of being in the sample.

Sampling with replacement (SWR)
In SWR, the first selected value is returned to the population and the second value is randomly selected from all N population values.
Sampling without replacement (SWOR)
In SWOR, the first selected value is removed from the population and the second value is randomly selected from the remaining N - 1 population values, etc.

Since a SWR may contain the same population value more than once, it covers less of the population than SWOR. SWOR therefore gives more accurate estimates of population characteristics.

Large populations and/or small samples

If the sample size, n, is much smaller than the population size, N, there is little practical difference between SWR and SWOR — there would be little chance of the same individual being picked twice in SWR.