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SAMPLING DESIGN

In simple words, sampling consists of obtaining information from a portion of a larger group or an universe. Elements are selected in a manner that they yield almost all information about the whole universe, if and when selected according to some scientific principles and procedures.

What is sampling ?

Helps to collect vital information more quickly and it helps to make estimates of the characteristics of the total population in a shorter time

Sampling cuts costs. Much of time and money is saved at each stage of research

Sampling techniques often increases the accuracy of the data. With small samples it become easier to check the accuracy of the data.

From the administrative point of view also sampling become easier – problem of hiring the staff, task of training and supervising will become easier

Advantages of sampling

Sampling is not flexible in a situation where knowledge about each unit is needed. E.g. estimation of national income for the current year.

Reliability of information depends upon the representativeness of the sample of the total population

Most of the sampling techniques require the service of a sampling experts or statisticians.

Disadvantages of sampling

Probability sampling techniques

Simple random sampling

Stratified random sampling

Systematic sampling

Accidental samples

Quota samples

Purposive samples

SAMPLING TECHNIQUESNon-probability sampling techniques

Random sampling mean, the arrangement of conditions in such a manner that every item of the whole universe from which we are to select the sample shall have the same chance of being selected as any other item.

Among all the probability sampling procedures random sampling is the most basic and least complicated.

Simple random sampling :

It involves listing all the elements in the population and assigning them consecutive numbers from 1 to n

Deciding upon the desired sample sizeUsing any method of selection, a certain

number of elements from the list of elements 1 to n is selected

One can interview the people or measure the item designated by the sample elements

Steps of a simple random sampling

Lottery method : here the no of all the

elements in the universe are written on different pieces of papers of equal size, shape and colour which are then shuffled thoroughly . Then the tickets are drawn at random , their numbers noted and corresponding individuals or objects are studied. The number of tickets drawn are equal to the sample size required. The process is thus designed to maintain objectivity and avoid any type of preference or bias towards any one unit.

Methods of drawing sample in Random Method

Tippet’s numbers : Tippet developed a list of

10,400set of numbers randomly, each set being four digits. These numbers were written on several pages in unsystematic order.

Now if we want to select a sample of 30 students from a population of 5000 students of Tamil Nadu, who matriculated this year. For the purpose of selection through Tippet’s method we have to follow the steps given below

Methods of drawing sample in Random Method

Prepare a complete list of universe. Every name has to be written in systematic order. Number it; in this case we can take the roll number of students.

Select any 30 numbers from any page of Tippet’s list systematically. We can pick up the first two or first three or last two or last three digits of each of the 30 sets selected from the Tippet’s list. Comparing our prepared list with these selected list we can select those persons who carry the same number.

Suppose for the universe of 5000 students we decide to make selection on the basis of first three digits then the selection process can be,

237 0 748 3 056 0 524 6 275 4 914 3 664 1 399 1 952 4 154 5Then the students carrying the numbers like

237,056,275,..etc would come under the sample.

Grid method : This method is applied in selection of

areas. For example we have to select any number of districts from Indian states. For selection first a map of whole area is prepared. The area is often divided into blocks. A transparent plate is made equivalent to the size of the map that consists of several squared holes in it which carry different numbers. By random sampling method it is decided to which numbers are to be included in the sample. Then the grid is placed over the map and the areas that come under the holes carrying selected numbers are included in the sample.

1. Prepare a list of all the elements in the universe and number them. This list can be according to alphabetical order, as in records etc.

2. Then from the list, every third/every 8th / or any other number in the like manner can be selected.

For this method, population needs to be homogeneous. This method is frequently used, because it is simple, direct and inexpensive. Also known as patterned, serial or chain sampling.

Systematic sampling Method

When the population is divided into different

stratas or groups and then samples are selected from each stratum by simple random sampling procedure, we call it as stratified random sampling.

Primary purpose of the stratified sampling method is to increase the representativeness of the sample without increasing the size of the sample on the basis of having greater knowledge of the population characteristics.

Stratified Random Sampling Method

According to the nature of the problem relevant criteria are selected for stratification. The possible stratifying criteria are age, sex, family, income, no. of years of education, occupation, religion, race, place of residence and so on.

Next step is to place every member of the population into its proper stratum as determined by the criteria used for stratification.

Elements are then selected from each stratum through simple random sampling method. An estimate is made for each stratum separately. These estimates are combined to provide an estimate for the entire population.

Steps :

Disproportionate stratified sampling: Also known as ‘equal size stratified

sampling’. In this method an equal no. of cases are selected from each stratum, irrespective of the size of the stratum in the universe.

Proportionate stratified sampling: Here the cases are drawn from

each stratum in the same proportion as they occur in the universe. Therefore, in this method the no. of samples to be drawn varies from stratum to stratum according to their size.

Types of Stratified Sampling

To apply the proportionate stratified sampling method we need to have a list of all the stratums. We would also need to know the proportionate size in the total population. Then we would select a given percentage of people from each stratum. Since the size of the stratums vary the no of persons coming from each stratum in the sample on the basis of selection of a given percentage of people will also vary.

Optimum allocation stratified sampling: In this design the object is to allocate parts of

the total sample size to the various sample strata so that the strata with the larger variances receive the larger no of sample cases.

Types of Stratified Sampling

A Non-Probability sampling is one in which one can not estimate beforehand the probability of each element being included in the sample. Also that it does not assure that every element has a chance of being included. The major forms of non-probability samples are

1. Accidental samples2. Quota samples3. Purposive samples

Non-probability Sampling

It means selecting the units on the basis of easy approaches. Here one selects samples that fall to hand easily. Suppose one is studying political socialization and political participation among the university and college students of Patna University and that his sample size in 100. He would go to the University campus and would select the first 100 students whom he happen to meet, whether in the class room, or in the student’s common room or in the football field. Such type of sampling is easy to do, and saves time and money. But the chance of bias are also great.

Accidental samples

In the quota sampling the interviewers are instructed to interview a specified no of persons from each category. In studying peoples status, living conditions, preference, opinions, attitudes, etc. the relevant basis of stratification usually used are: age, sex, education, place of residence, socio-economic status, religion and so on. The required no of elements from each category are determined in the office ahead of time according to time according to the no of elements in each category. Thus, an interviewer would need to contact a specified no of men and specified no of women, a certain no of persons from different age categories, from different religious or racial groups and so on.

Quota sampling

In purposive sample one picks up the cases that are considered to be typical of the population in which one is interested. The cases are judged as typical on the basis of the needs of the researcher. Since the selection of elements is based upon the judgment of the researcher, the purposive sampling is called judgment sample. The researcher tries his sample to match the universe in some of the important known characteristics. Defect with this method is that the researcher can easily make error in judging as to which cases are typical.

Purposive Sampling

Devendra Takur, Research Methodology in Social Sciences, Deep & Deep Publications (1993), ch:5

Reference