Sampling theory is a technique that is used by clinicians in conducting research in nursing or clinical matters. A group of people who share a common clinical condition or disease is collectively referred to as a target population. For the purpose of research, a smaller number of individuals is usually selected from this target population and they are referred to as a sample population. Sampling theory is hence defined as the process by which a sample population is selected from a target population. There are two main types of sampling theory: probability sampling method and non-probability sampling method. In probability sampling method, all subjects in the target population have an equal chance to be selected in the sample while in non-probability sampling, the sample population is selected in a non-systematic way (Elfil & Negida, 2017).
To illustrate this, take an instance whereby clinical nurses want to conduct a research on the effects of a disease like a stroke to its victims so as to establish better ways of handling the same, they will use sampling theory. Practically, there are many individuals all over the world who suffer from this ailment and the research cannot be done on all these people. It would thus be necessary for the researchers to identify a part of this population to be used in their study. In another instance, if some researchers want to do a study on the number of people suffering from Hepatitis B in Egypt, they may base the study on two major hospital facilities to reflect the situation in the entire country (Elfil & Negida, 2017).
Generalizability can be described as the process of making predictions based on past observations. In nursing research, generalizability is applicable to two main aspects of qualitative and quantitative research (Elfil & Negida, 2017).In the case of the quantitative aspect, clinicians use an observed population sample data to infer the results to the larger target population. In the study case given above whereby researchers wanted to establish the number of people suffering from Hepatitis B in Egypt, the study was conducted in only two medical facilities. The results observed here were then used to infer the case in the whole country using quantitative generalizability.
Generalizability in qualitative aspect involves the use of observations to give clinical hypothesis or theories. An instance of qualitative generalizability is the case whereby in the twentieth century, researchers examined a certain number of cigarette smokers in hospitals. The data collected from this study showed that more than 95% of the smokers had contracted lung cancer (Elfil & Negida, 2017). In this regard, researchers concluded that cigarette smoking causes lung cancer which is a theory that is recognized to date.

  Elfil, M., & Negida, A. (2017). Sampling methods in Clinical Research; an Educational Review. Emergency, 5(1), e52.