Per Course-notes.org the sampling theory is defined as a field of statistics that is involved with the collection, analysis and interpretation of data collected from random samples of a population

Per Course-notes.org the sampling theory is defined as a field of statistics that is involved with the collection, analysis and interpretation of data collected from random samples of a population. In other words, if we want to study all people with schizophrenia in Arizona, how would we do it? It is nearly impossible to study all of them- or the entire population that have schizophrenia, therefore, only a small part of the population is going to be studied, this is a sample.
Generality can be defined as the research findings and conclusions from a study conducted on a sample population applied to the population at large. In other words, generalizability depends on the degree to which the sample result can represent of the population. As an example, the researcher studies a sample of people with schizophrenia from Arizona than applies the results to a larger population with schizophrenia. Or a nurse studies a sample of population who had a fall in Progressive Care Unit in the last 5 years and applies the results to the entire population admitted to PCU.
Some requirements of generality are the need to have a study sample that represents some population of interest in our case people with schizophrenia, to understand the contexts in which the studies are done and how these studies influence the results. However, it does not matter how carefully researchers are during the research, there is no absolute guarantee that the results obtained in a study will occur in every situation outside the study.
Banerjee, A. & Chaudhury, S. (2010). Statistics without tears: Populations and samples . Industrial Psychiatry Journal, 19, 60–65. doi: 10.4103/0972-6748.77642
Course-notes.com. (2017). Sampling Theory. Retrieved from http://www.course-notes.org/statistics/sampling_theory