Which of the following is NOT a form of non-probability sampling?

Stratified sampling is the correct answer because it is a form of probability sampling. In stratified sampling, the population is divided into subgroups, or strata, that share similar characteristics. Then, random samples are drawn from each of these strata. This method ensures that every subgroup is adequately represented in the sample, which minimizes bias and allows for more accurate generalizations about the population.

In contrast, the other sampling methods listed—convenience sampling, quota sampling, and snowball sampling—are forms of non-probability sampling. In convenience sampling, the researcher selects participants based on their easy availability. Quota sampling involves selecting individuals to meet certain predetermined characteristics but does not use a random selection process. Snowball sampling relies on existing study subjects to recruit future subjects, which can lead to selection bias. Each of these methods lacks the randomization aspect that characterizes probability sampling, making stratified sampling distinct in its approach to ensuring representation in the sample.