The General Social Survey (GSS), conducted by the National Opinion Research Center at the University of Chicago, is a major source of data on social attitudes in the United States. Once each year, $1500$ adults are interviewed in their homes all across the country. The subjects are asked their opinions about sex and marriage, attitudes toward women, welfare, foreign policy, and many other issues. The GSS begins by selecting a sample of counties from the $3000$ counties in the country. The counties are divided into urban, rural, and suburban; a separate sample is chosen at random from each group. This is a

(a) simple random sample.

(b) systematic random sample.

(c) cluster sample.

(d) stratified random sample.

(e) voluntary response sample.

Number of adults interviewed $=1500$

Total number of countries $=3000$

The countries are categorized into three categories: urban, rural, and suburban.

A simple random sample (SRS) of size $n$ is made up of $n$ people chosen from the population with an equal chance of being the sample that is actually chosen.

The stratified sampling method divides the population into subgroups depending on criteria such as gender and geographic location. This is done in order to proportionally incorporate each attribute in the sample. In this example, the countries (populations) are separated into three groups based on geographical regions, and a sample is taken from each group to represent each of the groupings, after which they are asked for their opinions. As a result, stratified random sampling is employed.