A simple random sample is a subset of a statistical population in which each element of the subset has an equal probability of being chosen.
Systematic sampling is a random sampling method which is normally chosen by researchers for its clarity and its periodic quality.
In cluster sampling mentions to a type of sampling method where the researcher splits the population into different groups are known as clusters. Then, a simple random sample of clusters is chosen from the population.
In stratified sampling, the researcher splits the population into separate groups, called strata. Then, a probability sample is selected from each group.
A convenience sample is one of the main types of non-probability sampling techniques that is made up of people who are convenient to reach.
A voluntary sample is made up of people who are self-selected by researcher into the survey.
Since, Adam walks to the nearest high school (convenient to reach) and gives a survey postcard to every twentieth (i.e. systematic) student that enters the building, also he is free to select students for survey hence it is voluntary.
Therefore, the sampling method used here is convenience , voluntary and systematic.
