Sampling
Probability sampling (simple random, stratified, cluster) gives every member of the population a known chance of selection and supports generalisation. Non-probability sampling (convenience, purposive, snowball) is common in qualitative work, where depth matters more than representativeness.
Justify your sample size: a power analysis for statistical tests, or saturation and the scope of the question for interviews. State who was invited, who took part and who dropped out.
Example
Stratified random sample of 240 students across four colleges vs. 14 purposively chosen department heads.
Exercise
Write two sentences in your methodology section explaining who your participants are and why.