If we collect information about all individuals in the population, the study is called complete count or complete enumeration. The word census is also used for the entire population study. In statistical studies the complete count is usually avoided. If size of the population is large, the complete count requires a lot of time and the lot of funds. The complete count is mostly difficult for various reasons. Suppose we want to make a study about the cattle in the cattle form in any country. We are interested in the average cost of their food for a certain period. We want to link their cost of food with their sale price. This is of course, an important study. It is very difficult to collect and maintain the information about each and every cattle in the farms. If at all 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 cattles.
A complete count of the human population is called population census. For example in any country, the first population census was conducted in 1950 and the second was conducted in 1960. The third census of population could not be conduct in 1970 because of agitations in the country. It was conducted in 1972. The fourth census was conducted in 1980. The fifth population census was conducted in 1995. A lot of information is collected about the human population through the population census conducted regularly after 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, level of education and illiteracy in the country, geographical distribution of the population, age and sex distribution of the population etc.