Statistical studies primarily depend on adequate and reliable information. The investigator first examines the ‘preliminaries to data collection’ before actually collecting data. This enables him to select the suitable method to be used for collecting data. Besides this, collection of data also requires a preconsideration of the type of enquiry to be undertaken. The investigator may go for a detailed and exhaustive enquiry or may decide for a limited or sample type of enquiry. Accordingly, there are two types of statistical enquiries.

(a) Census type of enquiry (Complete enumeration method)

(b) Sample survey (Partial enumeration method)

As the name implies, census enquiry refers to the complete enumeration of each and every unit of the ‘Universe’ or ‘Population’. In statistics, universe (population) refers to the aggregate of objects, animate or inanimate under study. It includes all the items possessing a common characteristics in a statistical enquiry. For example the population census implies the counting of each and every human being within the country. So in census enquiry the whole groups of items are to be surveyed.

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A population may be either finite or infinite. A finite population contains finite or countable number of observations. For example, the numbers of students in a particular class or the number of books in the college library are *finite population*. On the other hand, a population having an infinite or uncountable number of items is called an infinite population. The numbers of stars in the sky, or the number of fishes in the river are examples of infinite population. Again population may be categorised as ‘existent population’ or ‘hypothetical population’. A population consisting of definite or concrete observations is existent population. For example, all the students of a class constitute an existent population. But the population consisting of imaginary or hypothetical observations is ‘hypothetical population’. The outcome of the tossing of a coin, when the coin is tossed infinite number of times constitutes a ‘hypothetical population’. The census method makes a detailed and exhaustive study of the enquiry. Because it involves complete enumeration of all the items in the population.