Sampling Technique: Survey is an important research method used to acquire knowledge systematically from a context of human experience. But the entire context, i.e. the entire population of interest cannot be acquired always as because one can neither afford money or time nor resources for scientifically covering the entire universe. Hence a sample is chosen from the entire population to project the result of the sample surveyed to its universe. The method through which a sample is chosen from a population is known as “Sampling Technique”.
a) Definition: Sampling is a technique where the sampler selects some of the elements with the intention of finding out a conclusion about the total population from which they are taken. It may be defined as the selection of some part of an aggregate or totality on the basis of which a judgment about the aggregate or totality is made. In other words, it is the process of obtaining information about an entire population by examining only a part of it.
b) Need for Sampling: The need for sampling is felt due to the following reasons-
i) It is generally more economical in time, effort and money to use sampling;
ii) If sampling is conducted by trained and experienced investigators then sampling may enable more accurate measurements for a sample study.
iii) Sampling remains the only way when population contains infinitely many numbers.
iv) Sampling remains the only choice when a test involves the destruction of the item under study.
v) Sampling usually enables to estimate the sampling error and thus, assists in obtaining information concerning some characteristic of the population.
c) Characteristic of Good Sampling: If the sample results are to have any worth while meaning, it is necessary that a sample possesses the following essentials characteristics.
d) Types of Sampling: Sampling can be categorized into the following-
i) Nonprobability Sampling: Non probability sampling methods are those which do not provide every item in the universe with a known chance of being included in the sample. The selection process is at least partially subjective. Nonprobability sampling again can be categorized into the following types
* Convenient Sampling: A convenience sample is obtained by selecting “convenient” population units i.e. the peoples who are convenient to response.
* Judgment Sampling or Purposive Sampling: In this method of sampling the choice of sample items depends exclusively on the judgment of the investigator. In other words, the investigator exercises his judgment in the choice and includes those items in the sample which he thinks are most typical of the universe with regard to the characteristic under investigation.
* Quota Sampling: In a quota sampling, quotas are set up according to some specified characteristic such as based on income, age, political or religious affiliations and so on. In the next step within each quota the selection of sample items depends on personal judgment of the researcher. It is the most commonly used sampling technique in non probability category.
* Snowball Sampling: It is a technique in which an initial group of respondent is selected randomly, and then subsequent respondent are identified based on the referrals provided by the initial respondents.
ii) Probability Sampling: Probability sampling methods are those in which every item in the universe has a known chance or probability of being chosen for the sample. This implies that the selection of sample item is independent of the person making the study that is the sampling operation is controlled so objectively that the items will be chosen strictly at random. Probability sampling can be grouped into the following
* Simple or Unrestricted Random Sampling: Simple random sampling refers to that sampling technique in which each and every unit of the population has an equal opportunity of being selected in the sample. In simple random sampling which item gets selected in the sample is just a matter of chance – personal bias of the investigator does not influence the selection. To ensure randomness of selection one may adopt either the lottery method or consult table of random numbers. The advantages of simple random sampling includes-
- It requires only a minimum of knowledge of the population in advance;
- It is more representative of the population as compared to judgment sampling;
- It is free from personal bias and prejudice;
- The method is simple to use;
- The analyst can easily assess the accuracy of this estimate because sampling errors follow the principle of chance.
* Systematic Sampling: If a population can be accurately listed or is finite, systematic sampling technique can be used. The lists are firstly prepared in alphabetical, geographical, numerical or some other order. The items are then serially numbered. The first item is selected at random generally by following the lottery method. Subsequent items are selected by taking every nth item from the list.
* Stratified Sampling: In stratified sampling the population of the universe is divided into smaller homogeneous groups, or strata by some characteristic and form and from each of these similar homogeneous groups draw at random a predetermined number of units. The usual stratification factors are sex, age, socio, economic status, educational background, residence (urban or rural), occupation, etc. In the standardization of test and public opinion polls, the method of stratification is necessary.
* Cluster Sampling: In multi-stage or cluster sampling, the random selection is made of primary, intermediate and final (or the ultimate) units from a given population or stratum. There are several stages in which the sampling process is carried out. At first, the first stage units are sampled by some suitable method, and then a sample of second stage unit is selected from each of the selected first stage unit, again by some suitable method, which may or may not be the same as that of the first method. Further stages may be added as required.
e) Limitations of the Sampling: Despite the various advantages of sampling, it is not completely free from limitations. Some of the difficulties involved in sampling are stated as follows:-
i) A sample survey must be carefully planned and executed otherwise the results obtained may be inaccurate and misleading.
ii) If sampling is not conducted by qualified and experienced persons, the information obtained from sample survey cannot be relied upon.
iii) If the information is required for each and every unit in the domain of study a complete enumeration survey is necessary.