The answer is "Stratified sampling".
Stratified sampling alludes to a sort of testing strategy . With stratified sampling, the specialist isolates the populace into discrete gatherings, called strata. At that point, a likelihood test (regularly a straightforward arbitrary example ) is drawn from each gathering.
Stratified sampling has a few favorable circumstances over straightforward irregular examining. For instance, utilizing stratified examining, it might be conceivable to decrease the example measure required to accomplish a given exactness. Or on the other hand it might be conceivable to expand the accuracy with a similar example measure.