Final answer:
Study subjects in observational studies are selected based on the research design and methodology, utilizing various sampling methods including convenience, purposive, and random sampling methods like stratified and cluster sampling.
Step-by-step explanation:
Study subjects in observational studies are selected through different sampling methods based on the research goals and study design. In cross-sectional studies, researchers may use convenience sampling if they aim to collect data at a single point in time from a sample that is easily accessible, although this can lead to biased data. In case-control studies, purposive sampling could be used to select participants based on specific characteristics that are of interest to the study, such as a particular medical condition. Cohort studies, which follow a group of individuals over time, might employ random sampling methods like simple random, stratified, or cluster sampling to ensure a representative sample of the population.
For the provided examples, the soccer coach used a form of stratified sampling where the players are divided into strata based on age and then sampled from each stratum. The pollster's method resembles cluster sampling, involving all the personnel within selected clusters (companies). The educational researcher is using stratified sampling by ensuring equal representation from two different strata (female and male teachers).