Chapter 16

Primary Studies:

Selecting a Sample Population

Types of Research Populations

At least four different types of populations must be considered when preparing to collect data:•The results of the study should be applicable to the target population •The source population is a well-defined subset of individuals from the target population•The sample population is the individuals from the source population who are asked to participate•The study population is the members of the sample population who actually participate in the study

Types of Research Populations

Target Populations

• A well-defined study question identifies a target population to which the results of the study should apply.

• A target population might be quite narrow (like one wing of a long-term acute care hospital) or relatively large (like a whole country).

• Unless the target population is very small, measuring the entire target population or even randomly sampling from it may be impossible.

Source Populations

A source population (sometimes called a sampling frame) consists of an enumerated list of population members. For example: •All women with a breast cancer diagnosis in the past 2 years who are indexed in a particular cancer registry•All members of a professional sports league•All households within 2 miles of a particular nuclear power plant

Sample Populations

A source population is often much larger than the sample size required for a study. In this situation, only a portion of the source population is selected to serve as a sample population.

A variety of probability-based sampling techniques can be used to select a sample population.

FIGURE 16- 2 Examples of Types of Probability Sampling

Sample Population

Sometimes a non-probability-based convenience population can be selected based on the ease of access to those individuals, schools, or communities.

However, convenience sampling must always be used with caution. Convenient sample populations are often systematically different than the communities they are intended to represent.

Study Populations

• The study population will consist of the members of the sample population who can be located, who consent to participation, and who meet all eligibility criteria.

• A 100% participation rate is extremely rare. • A low response rate may result in nonresponse bias if

the members of the sample population who agree to be in the study are systematically different from nonparticipants.

Study Populations

A less than 100% participation rate is usually not a problem as long as the researcher:•Uses suitable and carefully explained sampling methods•Takes appropriate steps to maximize the participation rate•Recruits an adequately large sample size

Cross-Sectional Surveys

• The goal of most cross-sectional surveys is to describe a specific target population accurately.

• Convenience samples rarely result in a study population that is representative of the target population.

• Ideally, the researcher needs some way to confirm that the source population is similar to the target population and that the sample population is similar to the source population.

FIGURE 16-3

Population

Example for a

Cross-Sectional Survey

Case-Control Studies

• All cases must have the same disease, disability, or other health-related condition.

• The controls must be similar to the cases in every way except for their disease status, so cases and controls should be drawn from populations with similar demographics.

FIGURE 16-4 Population

Example for a Case-Control

Study

Cohort Studies

• Longitudinal cohort studies: the participants should be representative of the source and target populations → The requirements for longitudinal studies are similar to those for cross-sectional studies, since both study designs recruit population-based samples.

• Prospective / retrospective cohort studies: the exposed and unexposed should be drawn from similar populations → The recruitment of exposed and unexposed for cohort studies is like the recruitment of the cases and controls for case-control studies.

FIGURE 16-5 Population Example for

a Cohort Study

Experimental Studies

• Experimental studies require a source population that is reasonably representative of the target population.

• Safety is always the top priority in designing an experimental study. The risk of harm to participants can be reduced by selecting an appropriate source population and defining strict inclusion and exclusion criteria.

FIGURE 16-6

Population

Example for an

Experimental Study

Vulnerable Populations

• Vulnerable populations in health research include some people with poor health, some people with limited decision-making capacity, and members of some socially marginalized groups, among others.

• Despite the potential risks of including members of these populations in research studies, including them is the only way to study health issues in these groups.– Example: The health of prisoners can only be

studied by conducting research in prisons.

Vulnerable Populations

• Research conducted with members of vulnerable populations requires extra consideration of the potential risks of research to participants.

• The ability of every participant to provide informed consent free from coercion must be assured.

• Concerns about the increased risks of adverse effects from study participation must be addressed.

Community Involvement

• Some studies benefit from or require the participation and/or support of whole geographic, cultural, or social communities and their leaders.

• Community-based studies often work best when they use methods such as those developed for Community-Based Participatory Research.