sampling techniques and sample size calculation · non-probability sampling meaning the subjects of...

Sampling What, why & how

Dr Girija P. Kartha

SamplingWhat, why & how

What is sampling?“Sampling is the act, process, or technique of selecting a suitable sample, or a representative part of a population for the purpose of determining parameters or characteristics of the whole population.”


Why sampling?

Sampling is done because you usually

cannot gather data from the entire population.

SAMPLING…….

TARGET POPULATION

STUDY POPULATION

SAMPLE SAMPLE


HOW ?

SAMPLE SELCTION

SAMPLING METHODS

SAMPLE SIZE


SAMPLE SELCTION

Key factor in research design.

determine whether research questions will be answered

clearly identify the population of interest for the specific research

question.


SAMPLING METHODS

MAINLY TWO METHODS:

PROBABILITY SAMPLING

NON-PROBABILITY SAMPLING

SAMPLING METHODS

PROBABILITY vs NON-PROBABLILITY SAMPLING


BASIS FOR COMPARISON



Meaning

The subjects of the population

get an equal opportunity to be selected as a representative

sample.

Subjects chosen by

researcher. Hence, who

will be selected can

not be predicted





Alternately known as

Non-random sampling

Random sampling

Basis of selection

Randomly Arbitrarily

Research Conclusive Exploratory





Result BiasedUnbiased

Method Objective Subjective

Hypothesis Tested Generated

SAMPLING TECHNIQUES

Simple random sampling

o Basic, most commonly used and easiest method.

o Each unit of the population has an equal chance of

inclusion in the sample.

o Applicable when population is small,

homogeneous and readily available.

Probability Sampling:

Simple

Random

Mainly two approaches are used…

• Lottery method

• Table of random number

Simple random sampling

Simple

Random

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 37 75 10 49 98 66 03 86 34 80 98 44 22 22 45 83 53 86 23 51

2 50 91 56 41 52 82 98 11 57 96 27 10 27 16 35 34 47 01 36 08

3 99 14 23 50 21 01 03 25 79 07 80 54 55 41 12 15 15 03 68 56

4 70 72 01 00 33 25 19 16 23 58 03 78 47 43 77 88 15 02 55 67

5 18 46 06 49 47 32 58 08 75 29 63 66 89 09 22 35 97 74 30 80

6 65 76 34 11 33 60 95 03 53 72 06 78 28 14 51 78 76 45 26 45

7 83 76 95 25 70 60 13 32 52 11 87 38 49 01 82 84 99 02 64 00

8 58 90 07 84 20 98 57 93 36 65 10 71 83 93 42 46 34 61 44 01

9 54 74 67 11 15 78 21 96 43 14 11 22 74 17 02 54 51 78 76 76

10 56 81 92 73 40 07 20 05 26 63 57 86 48 51 59 15 46 09 75 64

11 34 99 06 21 22 38 22 32 85 26 37 00 62 27 74 46 02 61 59 81

12 02 26 92 27 95 87 59 38 18 30 95 38 36 78 23 20 19 65 48 50

13 43 04 25 36 00 45 73 80 02 61 31 10 06 72 39 02 00 47 06 98

14 92 56 51 22 11 06 86 88 77 86 59 57 66 13 82 33 97 21 31 61

15 67 42 43 26 20 60 84 18 68 48 85 00 00 48 35 48 57 63 38 84

`

Tab

le o

f ran

dom

nu

mb

er

oRandom Numbers can be generated in excel by clicking of the formula

Systematic Random Sampling

o Applicable when the population is large, scattered and not

homogeneous.

o There is an equal interval between each selected unit.

o Population size – N, desired sample size- n,

Sampling interval K=N/n

o Randomly select a number and then every Kth element

thereafter.

Systematic Random Sampling

• Simple & convenient.

• Easier to implement in the field.

• The time and labor is relatively

small.

Systematic

Stratified Random Sampling

• Applicable when population is not homogeneous.

• The population is first devided into homogeneous

groups (age, sex, area, classes) called Strata.

• Then a simple random sample is selected from each

stratum.

Stratified

Stratified random sampling

◼It gives more representative sample.

◼Characteristics of each stratum can

be estimated and comparisons made.

◼It gives greater accuracy.

Stratified

MULTISTAGE SAMPLING

20

• The selection is done in stages. Cluster sampling is an

example of multistage sampling.

• First stage, random number of districts chosen in all states.

• Followed by random number of talukas, villages.

• Then third stage units will be houses.

• All ultimate units (houses) selected at last step are

surveyed.

Multiphasic sampling

• Part of the information is collected from whole population and part from sub samples.

• Eg. 20 fever cases clinical examination & blood test - high ESR - Widal, MP etc. –Those found negative again subjected to other tests.

Cluster Sampling• A cluster is a randomly selected group.

• Applicable when units of population are natural groups or clusters (villages, wards, blocks, factories, schools, etc.).

• From the chosen clusters, 30 in

number, the entire population

is surveyed.

Cluster

Cluster Sampling

In cluster sampling, instead of selecting all the subjects from the entire population right off, the researcher takes several steps in gathering his sample population. Cluster

Non-

probability

Quota

Judgment

SnowballConvenience

Non-Probability Sampling Methods

◼ Convenience Sample

◼ The sampling procedure used to obtain those units or people most conveniently available

Convenience/Purposive Sampling

o Subjects selected because it is easy to access them.

o This method is also called CHUNK.

o Generally used for making pilot studies.

o Biases are maximum.

o Results obtained are unsatisfactoryin terms of drawing conclusions.

Convenience

Judgment Sampling

The choice of study units depends exclusively on the judgment of theinvestigators.

based on some character of sample members to serve a purposeo

Judgment

Quota Sampling

o In this, the population is divided into quotas according to some specific characteristics

(age, sex, religion, etc.) represented to the exact extent that the investigator desires

No. of study units selected within each quotadepend on personal judgment.

.

◼ Snowball Technique

• When the target population is hidden and or

hard to reach

such as commercial sex workers, drug addicts or a patients of a rare disease or happenings in a controversial event.

Snowball

◼ Snowball sampling

◼ Initial respondents chosen by probability or non-probability methods,

◼ Then additional respondents are obtained by information provided by the initial respondents

Snowball

Person 1

Friend/contact 1

contacts his/her

own friends/co

ntacts/

Friend/contact 2

contacts his/her own

friends/contacts/

Friend/contact 3

contacts his/her

own friends/co

ntacts/4 5 6 7 8 9 10

`11 12

RESEARCHERRESEARCHER HAS 3 CONTACTS

Each 3 contacts has another 3 contacts.

SNOWBALL SAMPLING

Snowball

GROUP ACTIVITYLet us try some questions…

a) Which sampling method you will choose to identify the alcoholics in a city ?.

b) I want to study on the thought process of students who prefer to do post graduation in your class Which method of sampling would you choose?

c) In a TB survey, MT was done in all cases. In all MT +ve cases X-ray chest was done .

In all X-ray +ve cases sputum examination was done.Which survey method was used?

GROUP ACTIVITYGROUP – A

Q.1 There are 100 employees in an organization. The HR deptt. wants to select employees for a team building activity. What sampling method will be ideal ?

Q. 2 A researcher wants to know the views of the people who attended a dance program about certain aspects of the program. Which method would be chosen?

GROUP ACTIVITYGROUP – B

Q.3 Government of India wants to evaluate effectiveness of immunization program in Gujarat. Which method can be chosen?

Q. 4 A researcher wants to know the reasons of choosing ethical hacking as a profession. How will you choose your sample?

GROUP ACTIVITYGROUP – C

Q.5 To evaluate the popularity of use of City Metro as a transportation system as per the different income groups on a randomly selected day. What will be the ideal method of choosing samples?

Q. 6 To analyse the views of the population of a village about mobile phone usage which sampling method would be ideal?

GROUP ACTIVITYGROUP – D

Q.7 A researcher wants to study the prevalence of depression among the female population of Surendranagar. Which method of sampling can be chosen?

Q. 8 A researcher wants to know the effectiveness of the family planning program by choosing 500 adults from 5 villages with total population of 10000 each. Which is ideal sampling method?

sampling techniques and sample size calculation · non-probability sampling meaning the subjects of...

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