Stratified sampling

Stratification is the process of dividing members of the population into homogeneous subgroups before sampling.

The strata should be mutually exclusive and collectively exhaustive

Then simple random sampling or systematic sampling is applied within each stratum.

It can produce a weighted mean that has less variability than the arithmetic mean of a simple random sample of the population. 

In computational statistics, stratified sampling is a method of variance reduction when Monte Carlo methods are used to estimate population statistics from a known population.