Population

In social scientific research, a population is the cluster of people you are most interested in; it is often the “who” that you want to be able to say something about at the end of your study. Populations in research may be rather large, such as “the American people,” but they are usually less vague than that. For example, a large study for which the population of interest is more generally “the American people” will likely specify which American people, such as adults over the age of 18 or citizens or legal permanent residents it is examining.

It is quite rare for researchers to gather data from their entire population of interest. This might sound surprising or disappointing until you think about the kinds of research questions that social workers typically ask. For example, let’s say we wish to answer the following research question: “How does gender impact success in a batterer intervention program?” Would you expect to be able to collect data from all people in batterer intervention programs across all nations from all historical time periods? Unless you plan to make answering this research question your entire life’s work (and then some), your answer is probably a resounding no. So, what to do? Do you have to give up your research interest because you don’t have the time or resources to gather data from every single person of interest?

Absolutely not. Instead, researchers use a smaller sample that is intended to represent the population in their studies.

Sampling frames

An intermediate point between the overall population and the sample that is drawn for the research is called a sampling frame. A sampling frame is a list of people from which researchers draw a sample. But where do you find a sampling frame? Answering this question is one of the first steps in conducting human subjects research. Social work researchers must think about locations or groups in which their target population gathers or interacts. For example, a study on quality of care in nursing homes may choose a local nursing home because it’s easy to access. The sampling frame could be all of the patients at the nursing home. You would select your participants for your study from the list of patients at the nursing home. An administrator at the nursing home would give you a list with every resident’s name on it from which you would select your participants. If you decided to include more nursing homes in your study, then your sampling frame could be all of the patients at all of the nursing homes you included.

The nursing home example is perhaps an easy one. Let’s consider some more examples. Unlike nursing home patients, cancer survivors do not live in an enclosed location and may no longer receive treatment at a hospital or clinic. For social work researchers to reach participants, they may consider partnering with a support group that serves this population. Perhaps there is a support group at a local church in which survivors may cycle in and out based on need. Without a set list of people, your sampling frame would simply be the people who showed up to the support group on the nights you were there.  In this case, you don’t start with an actual list; you have a hypothetical one.  The sampling frame only comes into existence after you go to the support group and collect names.

More challenging still is recruiting people who are homeless, those with very low income, or people who belong to stigmatized groups. For example, a research study by Johnson and Johnson (2014) attempted to learn usage patterns of “bath salts,” or synthetic stimulants that are marketed as “legal highs.” Users of “bath salts” don’t often gather for meetings, and reaching out to individual treatment centers is unlikely to produce enough participants for a study as use of bath salts is rare. To reach participants, these researchers ingeniously used online discussion boards in which users of these drugs share information. Their sampling frame included everyone who participated in the online discussion boards during the time they collected data. Regardless of whether a sampling frame is easy or challenging, the first rule of sampling is: go where your participants are.

selecting study participants

Once you have a sampling frame, you need to identify a strategy for sampling participants.  You will learn more about sampling strategies later in this chapter.  At this point, it is helpful to realize that there may be some people in your sampling frame that you do not ultimately to enroll in your study.  You may have certain characteristics or attributes that individuals must have if they participate in your study. These are known as inclusion and exclusion criteria. Inclusion criteria are the characteristics a person must possess in order to be included in your sample. If you were conducting a survey on LGBTQ discrimination at your agency, you might want to sample only clients who identify as LGBTQ. In that case, your inclusion criteria for your sample would be that individuals have to identify as LGBTQ. Comparably, exclusion criteria are characteristics that disqualify a person from being included in your sample. In the previous example, perhaps you are mainly interested in discrimination in the workplace and don’t want to focus on bullying in schools.  You might exclude individuals who have not worked, who are currently enrolled in school, or might even set an age limit to people who are legal adults and exclude people who are less than 18 years old.  Many times, exclusion criteria are often the mirror image of inclusion criteria.  This would be the case if the inclusion criteria included being age 18 or older and the exclusion criteria included being less than 18 years old.

At this stage, you are ready to recruit your participants into your study. Recruitment refers to the process by which the researcher informs potential participants about the study and attempts to get them to participate. Recruitment comes in many different forms. If you have ever received a phone call asking for you to participate in a survey, someone has attempted to recruit you for their study. Perhaps you’ve seen print advertisements on buses, in student centers, or in a periodical.  As you learn more about specific types of sampling, you can make sure your recruitment strategy makes sense with your sampling approach.

Sample

Once you recruit and enroll participants, you end up with a sample. A sample is the group of people you successfully recruit from your sampling frame to participate in your study. If you are a participant in a research project—answering survey questions, participating in interviews, etc.—you are part of the sample of that research project. Some social work research doesn’t use people at all. Instead of people, the elements selected for inclusion into a sample are documents, including client records, blog entries, or television shows. A researcher conducting this kind of analysis, described in detail in Chapter 10, still goes through the stages of sampling—identifying a sampling frame, applying inclusion criteria, and gathering the sample.

Applying sampling terms

Sampling terms can be a bit daunting at first. However, with some practice, they will become second nature.  The process flows sequentially from figuring out your target population to thinking about where to find people from your target population to finding a sampling frame of people in your population to recruiting people from that list to be a part of your sample. Through the sampling process, you must consider where people in your target population are likely to be and how best to get their attention for your study. Sampling can be an easy process, like calling every 100th name from the phone book one afternoon, or challenging, like standing every day for a few weeks in an area in which people who are homeless gather for shelter. In either case, your goal is to recruit enough people who will participate in your study so you can learn about your population.

In the next two sections of this chapter, we will discuss sampling approaches, also known as sampling techniques or types of samples. Sampling approach determines how a researcher selects people from the sampling frame to recruit into her sample. Because the goals of qualitative and quantitative research differ, so too does the sampling approach. Quantitative approaches often allow researchers to make claims about populations that are much larger than their actual sample with a fair amount of confidence. Qualitative approaches are designed to allow researchers to make conclusions that are specific to one time, place, context, and group of people. We will review both of these approaches to sampling in the coming sections of this chapter. First, we examine sampling types and techniques used in qualitative research. After that, we’ll look at how sampling typically works in quantitative research.