what is a stratified sample

Stratified sampling is a method of sampling from a population that can be divided into subpopulations or strata. The process involves dividing the population into homogeneous subgroups based on specific characteristics, such as age, gender, income level, or education. Each subgroup is then sampled using a probability sampling method, such as simple random sampling, to ensure that every characteristic is properly represented in the sample. This helps improve the precision of the sample by reducing sampling error and allows for more accurate estimation of population parameters for each subgroup. Stratified sampling is particularly useful when the population's characteristics are diverse and when researchers want to ensure that every characteristic is adequately represented in the sample, leading to a more representative and valid study. It is not useful when the population cannot be exhaustively partitioned into disjoint subgroups