Advanced Research Methods- Chapter 5: Selecting Research Participants

Population

The entire set of individuals of interest to a researcher.

Sample

The small set of individuals who participate in the study. Need to be a good representation of the population of interest.

Threats to selecting representative samples

Bias and Selection bias

Bias

Sample’s characteristics are noticeably different from those of the population. Can occur by chance.

Selection Bias

a.k.a Sampling Bias. Subjects are selected in a manner that increased the probability of a biased sample.

Law of large numbers

A large sample will probably be more representative of the population than a small sample. A sample size of 25-30 individuals/scores per condition is a good target.

Factors that influence sample size

Practical limitations: Lots of conditions require lots of participants. Unique population may limit sample size. Desired margin of error: Increase sample size to minimize error.

Ethical concerns of sample sizes

Small n’s may not have enough power. Large n’s may be overpowered.

Categories of sampling methods

Probability sampling and Nonprobability sampling.

Probability sampling

Each individual in the population has a specified probability of selection. Must be possible to list all individuals in the population. Must be random, as it is more likely to yield representative samples.

Nonprobability sampling

Odds of selecting a particular individual are unknown. The sample may or may not be representative of the population. ????????????????