Summary
Highlights
A population is the entire group you want to draw conclusions about, while a sample is the smaller group of individuals you'll actually collect data from. Defining your population precisely makes it easier to gather a representative sample. For instance, instead of all high school students in the US, narrowing it to 9th-grade students in low-income areas of New York makes the research more manageable.
There are two main approaches to selecting a sample: probability sampling and non-probability sampling. Probability sampling uses random methods and is mainly used in quantitative research, allowing for strong conclusions about the whole population. Non-probability sampling uses non-random methods, is almost always used in qualitative research, and can also be used in quantitative research, but carries a higher risk of bias.
Probability sampling ensures a representative and unbiased sample, allowing for strong statistical conclusions. Methods include simple random sampling, systematic sampling, stratified sampling (dividing into subgroups and random sampling from each), and cluster sampling (dividing into geographical areas and randomly selecting some clusters). This approach often requires a list of all potential subjects, making it difficult for large populations.
Non-probability samples are easier to achieve but carry a higher risk of bias. If a sample is chosen based on convenience, accessibility, or volunteers, it might differ systematically from the overall population. For example, high academic achievers are more likely to volunteer for a study, leading to biased results. Researchers should be aware of limitations and potential biases when using these methods.
In some qualitative designs, such as ethnography or case studies, sampling a population may not be relevant. The goal is often to deeply understand a specific context rather than generalize. Instead, researchers focus on collecting as much data as possible about the chosen context, with a clear rationale for why a particular case or community is suitable for answering the research question.