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Sampling

Samples are a defined subset of the population chosen to represent the population under study. It is often impractical to conduct a survey, interview, or focus group with an entire population due to time and cost constraints. Sampling a subgroup of the population can overcome these limitations and make it possible to draw conclusions that are generalizable to the overall population.  

Sample selection

The ability to generalize from a sample to the population depends on the representativeness of the sample.   A sample is representative to the extent that it exhibits the same distribution of characteristics as the population from which it was selected.    For example, is your population all students enrolled in your department's courses or only those students who are majors? You may also wish to use sampling techniques to ensure that certain subgroups of your population are included.

Types of sampling

With proper sampling techniques, a researcher can use the information obtained from the population to infer something about the characteristics of the population.

Probability sampling

Each member of the population has a specifiable probability of being chosen. Probability sampling is critical when you want to make precise statements about a specific population on the basis of the results of your survey.

Nonprobability sampling

With nonprobability sampling, there is no way to estimate the probability that each member of the population will be included in the sample and there is no guarantee that each member of the population will have the same chance of being included.

Sample size

However, even if you carefully apply sampling strategies, some error may result from chance variation. This error can be reduced by increasing the sample size. When performing quantitative analysis, you must to choose a sample size that is appropriate for the statistical test you plan to use. Larger samples are more likely to yield results that are more reliable, precise, and representative of the population. Sample size may also depend on the resources available (time, money, staff) and how difficult it is to locate respondents. The table below provides a general guide for determining sample size.  Because your response rate is likely to be less than 100%, be sure to administer your survey to a group larger than your desired sample size to. [more]

Recommended sample size
Population size Recommended sample size

50

44

75

63

100

80

150

108

200

132

300

168

400

196

500

217

1000

277

3000

340

5000

356

10000

369

Additional information

Kalton, G. (1983). Introduction to Survey Sampling. Newbury Park,CA: Sage Publications, Inc.

Page last updated: Sep 21 2011
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