🧪 Selecting Research Participants

What is a population?

The entire group of interest (e.g., all adults, all humans).

What is an accessible population?

The portion of the population that can actually be studied.

What is a sample?

A smaller subset of the population that participates in the study.

What is the goal of sampling?

To create a sample representative of the population for generalization.

What does the law of large numbers state?

Larger samples provide more accurate estimates of population characteristics.

What is random (probability) sampling?

Every individual has a known chance of selection; reduces bias and improves external validity.

What is simple random sampling?

Selecting individuals randomly from a complete list of the population.

What is systematic sampling?

Selecting every kth person after a random start.

What is stratified random sampling?

Sampling equal numbers from identified subgroups (e.g., gender, race).

What is cluster sampling?

Randomly selecting entire groups (e.g., classrooms, neighborhoods).

What is multilevel sampling?

Sampling at multiple hierarchical levels (e.g., states → counties → neighborhoods → houses).

What is non-random (nonprobability) sampling?

Population size or selection probability is unknown; easier but more biased.

What is convenience sampling?

Selecting participants who are easiest to reach (e.g., college students, MTurk workers).

What is quota sampling?

Filling subgroup quotas within a convenience sample.

What is volunteer bias?

Systematic differences between volunteers and non-volunteers that threaten external validity.

What is the key goal of good sampling?

Reduce bias and improve generalizability.

Which sampling method has the highest external validity?

Proportionate stratified sampling.

Which sampling method is easiest but most biased?

Convenience sampling.