Definition: The Population Distribution is a form of probability distribution that measures the frequency with which the items or variables that make up the population are drawn or expected to be drawn for a given research study.
The characteristics or attributes of the population, i.e. the value of each variable in the population can be determined only when the investigator completely enumerates all the items of the population using the census method. By doing so, the frequency of these characteristics, i.e. probability of selecting some particular characteristic from the population, can be determined.
In case the population size is large and its complete enumeration is not possible, then the representative samples can be selected from the population. By doing so, several cases falling in several classes or categories can be observed to determine the frequency with which the cases of particular classes are likely to be drawn from the sample.
Once the complete information about the population is gathered, it is believed that the investigator has the knowledge of the population mean and standard deviation. For example, if a company has manufactured 5,000 cars in 2013 and want to gather information about those who had bought it and their experience so far. On the basis of such information, we can compute the population means (?) and standard deviation (σ).