Selecting Research Participant 1

Sample & Population A population is the entire set of individuals of interest to a researcher. A sample is a set of individuals selected from a population and usually is intended to represent the population in a research study. 2

Selection bias A representative sample is a sample with the same characteristics as the population. A biased sample is a sample with different characteristics from those of the population. Selection bias or sampling bias occurs when participants or subjects are selected in a manner that increases the probability of obtaining a biased sample. 3

Sample Size The first principle is that a large sample is probably more representative than a small sample. Although large samples are good, there is also a practical limit to the number of individuals that is reasonable to use in a research study. 4

Sample Size Although a sample size of 25 or 30 individuals for each group or each treatment condition is a good target, other considerations may make this sample size unreasonably large or small. It can be computed that for a population of 100,000 or more the sample must have at least 384 individuals to be confident that the preferences observed in the sample are within 5% of the corresponding population preferences. 5

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Sampling Basics Sampling methods fall into two basic categories: probability sampling (5 types) nonprobability sampling.(2 types) 8

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