One of the important steps for researchers to conduct their study is the recruitment of research participants. Given this, the researchers need to identify the following elements of research.

Inclusion/exclusion criteria: Prior to the initiation of research, the researcher should define the characteristics of the prospective participants of research. These characteristics are called inclusion criteria, which provide the chance for the determination of population. On the other hand, exclusion criteria refer to the features, which set the boundaries for the ineligible participants.

Population or target population: It refers to a group of eligible individuals/cases/things to whom/which the researcher wish to generalize his research outcomes. Accordingly, the individuals/cases/things in the target population meet at least one similar characteristics.

Sampled population: Once the researchers determined the target population, they should decide whether it is possible to observe all the individuals in the population. Therefore, the sampled population is the population that is accessible to the researchers. The Participants in the sampled population are selected based on the inclusion/exclusion criteria.

Sample: Each individual selected based on a sampling technique is called a sample. Sampling is closely related to the two concepts of internal and external validity. External validity is the validity of generalizing the research outcome to target population. On the other and, internal validity avoids the confounding variables. Establishment of severe inclusion/exclusion criteria can lead to higher level of internal validity; however, it results in lower level of external validity.

Data collection: data collection can be performed in two ways, namely census and sampling. The former refers to the study of all individuals in the population, whereas the latter accounts for some individuals (samples) selected from the target population.   

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