dividing the target population into important subcategories, selecting members in proportion that they occur in the population. e.g. 25% of British are of Indian origin, so 25% of your sample should be of Indian origin... and so on
advantage
a deliberate effort is made to make the sample representative of the target population, more representative than opportunity sampling as there is equal representation of subgroups
disadvantage
it can be time consuming as the subcategories have to be identified and proportions calculated, as it uses random or opportunity sampling it still has the weakness of these methods
