Theory of SamplingPrepared by:

Jyotika Minhas

Assistant Professor

Department of Economics

Hans Raj Mahila Maha Vidyalaya

Jalandhar

Punjab

INTRODUCTION

For statistical enquiry we need reliable and

accurate data.

There are two ways to collect statistical

data.

Methods of enquiry

Census

method

Sampling

method

Universe or Population

Population or Universe in statistics is the

entire body of terms or items, about which we

want to obtain information.

TYPES OF UNIVERSE

Finite Infinite Existing Experimental

CENSUS METHOD

Under this technique each and every unit of

population is studied.

For example:- suppose we want to calculate the average income

of a colony.There are 10 houses in the colony and their total income are as follows:-

200,300,200,300,500,200,300,500,200,300

Their average income will be 300rs.

Merits of census method

Intensive study of Population

High Degree of accuracy and reliability

No chance of bias

Suitability of the method

Study of each and every unit of population

Demerits of census method

Time consuming

Costly method

Possibility of errors

Destructive population

Not suitable for infinite universe

Difficult to arrange large no. of trained

enumerators

This method is suitable when;

Universe is comparatively small.

Trained enumerators are available.

Intensive study is to be undertaken.

Accurate and exact results are required.

Sampling method

Sample:- A sample is a part of the universe or

population which contains the qualities of

universe.

In the sampling method only a part of

population is studied and on its basis valid

conclusions are made.

Examples:-

Basis of sampling

1. The law of statistical regularity:- if a large sample is

taken randomly from a population then the obtained

results are fairly closed to the census method.

2.The law of intersia of large numbers:-according to this

law ,larger is the size of sample,more chances of its

results are likely to be near to that of population.

Properties of good sample

Representativeness

Adequacy

Independence

Homogeneity

Sample should be in accordance with the

objective of investigation

Advantages of sampling method

Saves time and labour

Economical method

More dependable results

More detailed information can be collected

Destructive population

Sample investigation is easy to organise and

supervise

More scientific than census

Disadvantages of sampling method

Not suggested in all circumstances

It is not possible to achieve hundred percent

accuracy

Requires services of experts

May not be always economical

Difficult to obtain representative sample in

practice

Sampling errors

Difference between census method

and sampling method

1. Degree of accuracy

2. Scope

3. Cost

4. Convenience

5. Time

6.Reliability

7.Nature of items

8.Principles

9.Verification

10.Area

METHODS OF

SAMPLING

RANDOM SAMPLING

METHOD

In this method each and every

Unit has an equal or some definite

probability of being selected in a

sample.

Probability Sampling Methods

a) Simple Random Sampling

b) Restricted Random Sampling

I. Stratified sampling

II. Systematic sampling

III. Cluster sampling

Selection of Simple Random Sample

1. Lottery Method

2. Random Number Tables

Merits

1. There is no chance of

personal biasness.

2. It saves lot of time and

money.

3. It is a scientific method.

Demerits

1. Random sampling is not suitable for

heterogeneous data.

2. In lottery method if slips are not made

identically, so there is chances of

biasness.

3. The investigator has no control over the

selection of units.

Stratified Random sampling

Under this method the

universe is divided

into sub groups and

sample is taken from

each sub group. The

Sub group is known

as strata.

Methods of Stratified

sampling

1. Proportional method

2. Disproportional method

Merits

1. It helps in achieving high degree of accuracy.

2. There is a greater control of investigator in this method.

3. They are expected to be localised in a small area . So it is convenient method.

Merits

Demerits

1. There is possibility of biasness.

2. It is more expensive.

3. It is very difficult task to divide the

universe into homogeneous groups.

Systematic Random

Sampling

Under this method a sample is

taken from a list prepared

on a systematic arrangement

such as alphabetically ,

chronologically,

geographically order .

Merits

1. It is easy to understand.

2. It saves lot of time , energy and

finance.

3. If size of population is large then this

method is suitable.

Demerits

1. To apply this method a complete and

up to date list of units is required, which

is usually not available.

2. Any hidden periodicity in the list will

adversely affect the representative

character of the sample.

•This type of sampling is done in stages.

•Under this sampling the population is divided and sub – divided into groups called

clusters and sampling process is accomplished in

different stages.

Merits

1. This method is simple to understand.

2. Multistage sampling is more flexible

than other methods.

3. This method is systematic method

covering the divisions and sub-divisions

of entire population.

Demerits

1. If number of cluster large then

representative sampling is affected

2. This method requires higher cost and

time.

3. The results obtained through this

sampling is less accurate than simple

random sampling.

Non Probability Sampling is a technique

in which the selection of the sample

depends upon the personal judgement

of the investigator .

1. Judgement sampling

2. Convenience sampling

3. Purposive sampling

4. Quota sampling

TECHNIQUES OF NON PROBABILITY SAMPLING

In this process , sampling is

done on the judgement of

the investigator . This

method requires a deep

and thorough knowledge

of the universe on the part

of the investigator.

1. This method is applied when size of

sample is small.

2. If we have to study some of the known

characteristics of the universe then

judgement sampling helps us.

3. Judgement sampling is helpful in solving

day to day problems .

1. This method suffers from the personal judgement

of the investigator.

2. It is difficult to calculate sampling error because

the items are not based on random sampling

techniques.

3. It is a non-scientific method.

Convenience SamplingIn this method, the sample units are

selected as per the

Convenience of the investigator . He

selects only those

Units which are convenient for him.

Merits

1. This method is applied when

population is not clearly defined.

2. This method can be used when there is

non- availability of resource list.

Demerits

1. This is a non-scientific method.

2. Personal bias is there.

3. The results obtained are not true

representative of the population.

Quota Sampling

Under quota sampling the population is divided into some groups according to some characteristics like age , sex, occupation, religion, education level etc. Then investigator is then told to select from each of the sub-group according to some pre determined quota.

Merits 1. Quota sampling is often used in public

opinion surveys including all sections

of society.

2. In quota sampling , the cost of

sampling is less then other methods.

3. This method can give accurate results

if proper checks or control are

imposed.

Purposive Sampling

In such sampling the investigator has

some pre-determined objective In

his mind which he wants to impress

upon .Purposive sampling is also

known as Deliberate sampling.

Merits

1. The investigator may include necessary

items and remove the non-essential.

2. Such kind of sampling solves the purpose of

the investigator.

3. In this sampling, the investigator has full

control and can represent the samples in the

best possible way.

Demerits

1. It suffers from personal bias.

2. It requires complete knowledge of the

population , but investigator lack such

knowledge.

3. It is highly non-scientific in nature.

4. It does not represent the entire

population.