Sampling

Selecting Subjects for Survey Research…

The main purpose of survey research is to describe the characteristics of a population by collecting data from a sample.

POPULATION–> The population is the group consisting of all people to whom we (as researchers) wish to apply our findings. If we were interested in the reading level of 3rd graders in Connecticut, the population would be all third graders in Connecticut. The data (information) we collect from populations are called PARAMETERS and are said to be DESCRIPTIVE. We label the number of subjects (observations) in a population with an upper case N (N=300). The first step in sampling is to define the population (3rd graders in Connecticut). (The actual population to whom the researcher wishes to apply his or her findings is called the TARGET population.  Often the TARGET population is not available, and the research must use an ACCESSIBLE POPULATIONS. In this case, the researcher can only apply (generalize) his or her findings to that group.

SAMPLE–> Subsets of people are usually used to conduct studies. These subsets are called samples. The samples are used to represent the population from which they were drawn. The data we collect from samples are called STATISTICS and are said to be INFERENTIAL (because we are making inferences about the POPULATION with data collected from the SAMPLE). We label the number of subjects (observations) in a sample with a lower case n (n=25).


Statistics are used to effectively communicate numerical information to other people. In statistics we are…

…Looking at RELATIONSHIPS among (between) characteristics (i.e., salary & job satisfaction; food consumption & energy) — Correlation Research (which we study in a different unit) is an example of research involving relationships.

…Looking at DIFFERENCES between (among) groups (i.e., males & females; experiment & control) — Experimental Research (which we study in a different unit) is an example of research that looks at differences.

…Looking to DESCRIBE the characteristics of the population from data collected from a sample — Survey Research.   The two major types of surveys are cross-sectional survey and longitudinal survey (trend, cohort, and panel studies).


Inferential statistics are used to determine how likely it is that characteristics exhibited by a sample of people are an accurate description of those characteristics exhibited by the population of people from which the sample was drawn.

The term statistically significant (which we will study later, so don’t worry about it for this unit) (p < .05) is used merely as a way of indicating the chances are at least 95 out of 100 that the findings obtained from the sample of people who participated in the study are similar to what the findings would be if one were actually able to carry out the study with the entire population.


The first step in selecting a sample is to define the population to which one wishes to generalize the results of a study. Unfortunately one may not be able to collect data from his or her TARGET POPULATION. In this case, an ACCESSIBLE POPULATION is used. If the latter is used, care must be taken not to generalize beyond the ACCESSIBLE POPULATION.

  • -The sample is drawn from the population
  • -Data is collected from the sample
  • -Statistics are used to determine how likely the sample results are reflective of the population

A number of different strategies can be used to select a sample. Each of the strategies has strengths and weaknesses. There are times when the research results from the sample cannot be applied to the population because threats to external validity exist with the study. The most important aspect of sampling is that the sample represent the population.

*CHOOSING A SAMPLE*

Sample Size
How large should my sample be? Large enough to be an accurate representation of the populaton and large enough to achieve statistically significant results

Larger Samples are needed when…

  • a large number of uncontrolled variables are interacting unpredictably
  • the total sample is to be divided into several subsamples (the researcher is interested in also studying subgroups within the sample)
  • the population is made up of a wide range of variables and characteristics
  • differences in the results (effect size) are expected to be small
  • high attrition of subjects is expected

The number of subjects you select (use the sample size calculator to determine this) will influence how confident you can be that your results depict the population from which the sample was drawn.

 

Del Siegle, Ph.D.
Neag School of Education – University of Connecticut
del.siegle@uconn.edu

www.delsiegle.com