CHAPTER 6: SAMPLING

Populations and Samples Population: a group of individuals a researcher seeks to learn about from a research study. Sample: the group of individuals chosen from the population to represent it in a research study.  The actual size of the sample depends on the type of study and the resources that researchers have for collecting data from their sample. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Populations and Samples Sampling error: the difference between the observations in a population and in the sample that represents that population in a study. Probability sample: sample chosen such that individuals are chosen with a specific probability. Convenience sample: sample chosen such that the probability of an individual being chosen cannot be determined. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Probability Samples Individuals are chosen at random from the population such that the chance of any one individual being selected is known.  Probability samples are most likely to be representative of the population, so you should use these samples whenever it is feasible to do so. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Probability Samples Simple random sample: sample chosen randomly from the population such that each individual has an equal chance of being selected. Cluster sample: sample chosen randomly from clusters identified in the population. Stratified random sample: sample chosen from the population such that the proportion of individuals with a particular characteristic is equivalent in the population and the sample. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Simple Random Sample McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Cluster Sample McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Stratified Random Sample McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Convenience Samples Individuals are chosen non-randomly from the population such that available individuals are chosen and the chance of any one individual being selected is not known.  Thus, the sample may not represent the population as well as a probability sample but can be obtained more easily than a probability sample. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Volunteer Samples Sample chosen from the population such that available individuals are chosen based on who volunteers to participate. Volunteer samples are convenience samples because the probability of recruiting any individual from the population is not known due to a lack of random selection from the population or subset of the population. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Volunteer Sample McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Quota Samples Sample chosen from the population such that available individuals are chosen with equivalent proportions of individuals for a specific characteristic in the population and sample. The quota sample makes use of convenience sampling techniques such as recruiting participants from a participant sign-up pool or asking students sitting in the library to fill out the survey. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Probability vs Convenience The advantage of convenience samples is that they are easier to obtain than probability samples. The disadvantage is that they may not be truly representative of the population a researcher wants to learn about, lowering the external validity of a study. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Recruiting Participants Regardless of the type of sampling technique chosen, researchers must consider how to actually recruit the participants for a study. The issue of how to contact the participants to recruit them is related to the selection mechanism in some types of studies. The means of contacting participants is an important issue, as it may affect the sample size obtained for a study. Researchers should carefully consider recruitment techniques that may be most effective for their population. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Recruiting Participants Incentives can also have an impact on the sample size obtained. For established institution participant pools, there may be incentives built in to the recruitment process, such as completion of an assignment or extra credit in psychology courses. Other incentives such as payment or entrance in a drawing may be also be possibilities available to researchers if the resources for these incentives exist. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Internet Samples Samples obtained using the Internet are becoming more frequent in recent psychological research. Advantages  Use of larger and more diverse samples.  Fewer opportunities for bias due to experimenter interactions with participants.  Fewer constraints on the time and location of data collection, less time needed to collect a sample, and fewer researchers needed to supervise the study procedures. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.

Internet Samples Disadvantages  Some smaller subgroups in the population may require extra work to include in the sample because the number with access to the Internet may be too small to allow for adequate sampling.  It may be difficult to control who completes a study on the Internet, making stratified random and quota samples more difficult to obtain and making it difficult to monitor repeat participation from the same individuals.  It may also be difficult to balance group size in between-subjects designs.  It is more difficult to monitor participants during the study procedure to rule out sources of bias (such as distractions) for Internet samples. McBride, The Process of Research in Psychology. Third Edition © 2016 SAGE Publications, Inc.