Types of Population

In research, a population refers to the complete set of individuals, groups, organizations, events, objects, or other units that share particular characteristics relevant to a study and about which the researcher seeks to generate conclusions. Also known as the target population or universe, it constitutes the entire body of elements that fall within the scope of the investigation. A population is not limited to human beings; it may also include households, institutions, communities, documents, programs, case records, countries, or any other units of analysis that are pertinent to the research objectives.

The population serves as the foundation for data collection and determines the scope of the study. Researchers define the population based on the purpose of the investigation and use it as the basis for selecting a sample when studying every member is impractical. Clearly identifying the population is essential because it influences the research design, sampling procedures, data collection methods, and the validity of the study’s findings.

Types of Population

In research methodology—particularly within social work and the social sciences—”types of population” can be understood from several different angles. The most common and useful distinctions are based on the researcher’s relationship to the population, the size of the population, and the population’s physical existence.

There are several types of population in research, classified based on different criteria. The most commonly used classification includes:

       i.          Based on the Research Framework (The Most Important Distinction)

§  Target Population (Theoretical Population)

Target Population (Theoretical Population) refers to the entire group of individuals, objects, or units that a researcher is interested in studying and about which they intend to draw conclusions. It is defined conceptually based on the objectives of the research and represents the ideal or complete set of elements that share the relevant characteristics of the study. This population is often not fully accessible in practice, but it serves as the basis for generalization of research findings. For example, in a study on the mental health status of migrant workers in Nepal, all migrant workers across the country constitute the target or theoretical population, even though the researcher may not be able to reach every individual within this group.

§  Accessible Population (Study Population)

Accessible Population (Study Population) refers to the portion of the target population that a researcher can realistically reach and study within the available time, resources, and practical constraints. It is the actual group from which data is collected and is considered a practical subset of the target population. This population is defined based on accessibility, geographic limits, and availability of respondents. For example, in a study on the mental health of migrant workers in Nepal, the accessible population may consist of migrant workers living in selected districts such as Banke and Dang, or those who are available during the data collection period.

     ii.          Based on Size & Existence (Sampling Framework)

§  Finite Population

Finite Population refers to a population that consists of a limited and countable number of elements. In this type of population, every unit can be clearly identified and enumerated, making it possible to determine the exact size of the population. Finite populations are commonly used in social science research where the study group is well-defined and manageable. For example, all students enrolled in a particular university in a given academic year represent a finite population because the total number of students can be accurately counted.

§  Infinite Population

Infinite Population refers to a population that has an unlimited or uncountable number of elements, making it impossible to determine its exact size. In research, this type of population is often theoretical and is assumed when the number of observations or events is continuously generated or extremely large. Infinite populations are commonly used in statistical studies and probability-based research. For example, the number of possible throws of a dice or the continuous production of outcomes in a long-term manufacturing process can be considered as an infinite population because the total number of outcomes cannot be precisely counted.

§  Hypothetical (or Conceptual) Population

Hypothetical (or Conceptual) Population refers to a population that is defined theoretically based on ideas, assumptions, or research objectives, rather than an actually existing or easily identifiable group. It represents an abstract population that may not be directly observable but is used for the purpose of analysis, comparison, or generalization in research. This type of population is often used in experimental, statistical, or theoretical studies where the focus is on broader concepts rather than a specific real-world group. For example, in a study on the effectiveness of a new counseling approach, the hypothetical population may include all possible individuals who could experience similar psychological conditions, even though they cannot all be identified or accessed in reality.

   iii.          Based on Status Relative to the Study

§  Prospective Population

Prospective Population refers to a population that includes individuals or units that are expected to become part of the study in the future based on certain criteria or conditions. It is commonly used in longitudinal or follow-up studies where the researcher identifies participants in advance and observes them over a period of time to study future outcomes or changes. This population is not fully available at the beginning of the research but is gradually formed as time progresses. For example, in a study on the impact of prenatal care on child health, pregnant women registered in health facilities during the study period constitute the prospective population, as they are followed over time to observe future health outcomes of their children.

§  Retrospective Population

Retrospective Population refers to a population that is defined based on past records, events, or conditions that have already occurred before the start of the research. In this type of population, the researcher studies existing data or historical information to analyze outcomes, causes, or patterns. Retrospective populations are commonly used in case-control studies, historical research, and record-based investigations. For example, in a study on factors associated with maternal mortality, all maternal death cases recorded in hospitals over the past five years constitute the retrospective population, as the study is based on previously documented events.

Factors to Consider While Selecting a Population