In the first example, the population consists of all married couples in the United States. In the second example, the population consists of all adult women in the United States. It is often difficult to make observations on every member of a population about which one wishes to make a conclusive statement. For example, a researcher who wants to study the relationship of lung cancer and smoking in the USA does not have to investigate every member of the smoking population in the United States. The investigator resorts to using a sample of the population of all cigarette smokers in the United States.
Probability sampling is the fundamental basis for all survey research.
The basic principle of probability sampling is that a randomly selected, small percent of a population of people can represent the attitudes, opinions, or projected behavior of all the people, if the sample is selected properly. Based on the results of his investigation on the sample, the researcher makes generalizations to the population. It is very important that we select a representative sample of a population if our investigation is going to reflect the population accurately. You may perform a simple experiment to understand the scientific basis of sampling. To perform the experiment you need different color marbles or beans.