3.1.2 Census and sample method
Under the census or complete enumeration survey method, data are collected from each and every unit (person, household, field, shop, factory, etc., as the case may be) of the population or universe which is the complete set of items which are of interest in any particular situation. For example, if the average wage of workers working in fish processing industry in India is to be calculated, then wage figures would be obtained from each and every worker working in the industry and by dividing the total wages which all these workers received by the number of workers working in the industry, we would get the figure of average wage. Some of the merits of the census method are:
(i) Data are obtained from each and every unit of the population.
(ii) the results obtained are likely to be more representative, accurate and reliable.
(iii) It is an appropriate method of obtaining information on rare events such as areas under some crops and yield thereof, the number of persons of certain age groups, their distribution by sex, educational level of people, etc. This is the reason why throughout the world the population data are obtained by conducting a census generally every 10 years by the census method.
(iv) Data of complete enumeration census can be widely used as a basis for various surveys
However, despite these advantages the census method is not very popularly used in practice. The effort, money and time required for carrying out complete enumeration will generally be very large and in many cases cost may be so prohibitive that the very idea of collecting information may have to be dropped. This is more true of underdeveloped countries where resources constitute a big constraint. Also if the population is infinite or the evaluation process destroys the population unit, the method cannot be adopted.
Sampling is simply the process of drawing a sample the population under study. Thus, using the sampling technique instead of every unit of the universe only a part of the universe is studied and the conclusions are drawn on that basis for the entire universe. A sample is a subset of population units. The process of sampling involves three elements:
a. determining the sample size
b. Selecting the sample units,
c. collecting obtaining information from sampled units
The three elements cannot generally be considered in isolation from one another. Sample selection, data collection, and estimation are all interwoven and each has an impact on the others. Sampling is not haphazard selection—it embodies definite rules for selecting the sample. But having followed a set of rules for sample selection, we cannot consider the estimation process independent of it—estimation is guided by the manner in which the sample has been selected.
Although much of the development in the theory of sampling has taken place only in recent years, the idea of sampling is pretty old. Since times immemorial people have examined a handful of grains to ascertain the equality of the entire lot. A housewife examines only two or three grains of boiling rice to know whether the pot of rice is ready or not. A doctor examines a few drops of blood and draws conclusion about the blood constitution of the whole body. A businessman places orders for material by examining only a small sample of the same. A teacher may put questions to one or two students and find out whether the class as a whole is following the lesson. In fact there is hardly any field where the technique of sampling is not used.
It should be noted that a sample is not studied for its own sake. The basic objective of its study is to draw inference about the population. In other words, sampling is a tool which helps to know the characteristics of the universe or population by examining only a small part of it. The values obtained from the study of sample, such as the average and dispersion, are known as ‘statistics’. On the other hand, such values for the population are called ‘parameters’.