3. 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
5. 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.
6. 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
7. 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.
8. FIGURE 16- 2 Examples of Types of
Probability Sampling
9. 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.
10. 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.
11. 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
12. 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.
14. 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.
16. 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.
18. 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.
20. 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.
21. 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.
22. 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.
