If we collect information about all individuals in a population, the study is called a complete count or complete enumeration. The word census is also used for an entire population study. In statistical studies, a complete count is usually avoided. If the size of the population is large, a complete count requires a lot of time and a lot of funds. The process is difficult for a number of reasons. Suppose we want to perform a study about the cattle in a country. We are interested in the average cost of their food for a certain period. We want to link their food cost with their sale price. It is very difficult to collect and maintain the information about each individual cattle on each farm. Even if we are able to do it, the study may not be of much use. The desired information can be obtained from a reasonable sample size of the cattle.
A complete count of the human population is called population census. For example, in a given country the first population census was conducted in 1950 and the second was conducted in 1960. The third census of the population could not be conducted in 1970 because of conflict in the country, so it was conducted in 1972. The fourth census was conducted in 1980. The fifth population census was conducted in 1995. A lot of information has been collected about the human population through the population census conducted regularly every 10 years. The census reports give information about various characteristics of the population, e.g., the urban and rural population, the skilled and unskilled labor forces, the agriculture labor force and the industrial workers, the level of education and illiteracy in the country, the geographical distribution of the population, the age and sex distribution of the population, etc.