Answer
By getting a sample in which the number of individuals from each group from the sample is proportional to the number of individuals from each group of the population, we can do an effective survey.
Work Step by Step
Suppose that, in the population (American adults), 40% are Republicans, 40% are Democrats and 20% are Independents. If you decide to obtain a simple random sample of size 100, you must get 40 Republicans, 40 Democrats and 20 Independents. This is a representative sample of the population.
