Sample survey is the technique to study the universe on the basis of a sample. A sample is a finite subset of the population. It represents a particular characteristic of the entire population. For example, a doctor examines a few drops of blood to test the blood of a patient, we test the quality of a bag of rice by testing a few grains out of it. Here few drops of blood or few grains of rice constitute the samples. The number of units in a sample is called “the sample size”. Thus sample survey is a method of analyzing the characteristics of the entire population by studying a representative part of it.
Sampling techniques are often used to have knowledge about the properties of the population. It minimizes time, energy and monetary expenses of studying the peculiarities of the population. The sampling method is considered scientific and convenient for collecting data from a large population.
In sample survey selection of sample items is very important. Sample items should be selected carefully so that they truly represent the population. One can get reasonably accurate information from survey conducted properly.
Very often sampling method is preferred to census method of collecting data, because of the following reasons.
i) The sample method involves less cost than the census method. Because here only a part of the population is examined. So it is economical.
ii) Sample study saves time and provides quick result.
iii) Sampling method often provides more accurate information than the census method. Because here we survey only a few items of the population. Sampling is generally done by trained and experienced persons. It facilitates intensive study and getting detailed information about the population.
iv) To get approximate or aggregate results sampling is generally preferred to census method.
v) In case of large population, sample method is more suitable than census method for collecting information.
vi) Non-sampling errors can be better controlled in sample survey than in census method.
Therefore, sampling involves less time provides accurate informations in a scientific and economical manner in comparison to census method.
Sample method also has a number of drawbacks. Some of the important drawbacks of this method are given below.
i) If it is a question of deliberate selection, the result may be very much biased. This shall mislead the enquiry.
ii) All characteristics of the population may not be found in the samples drawn from the population.
iii) Information from sampling method are relatively less accurate than that from census method.
iv) Sample survey needs proper planning and execution by trained personnel. Otherwise it may give wrong results.
v) The law of inertia of large numbers, accuracy and approximation are less accurate in this method as compared to the census type of enquiry.
In spite of the above demerits the sampling method is gaining popularity day by day. This is primarily because the method is theoretically more correct and practically more convenient.