It may be difficult to get a random sample if the majority of the people being studied and an area are of a similar background or opinion.
This is extremely important when the population being studied is considered the "whole population" and the simple random method of sampling is being used. When a demographic group is being studied, having an area with similar background and opinions will be appropriate.
This is why it is so important to determine exactly what population is being examined. And yes, with any statistical analysis, the large the sample the more powerful the statistic.
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