A sample is a subset of individuals from a larger population. Sampling means selecting the group that you will actually collect data from in your research.
2. Sampling is process of selecting representative
units from entire population of a study. In research
studies , it is not always possible to study an entire
population therefore the researcher draws a
representative part of population through sampling
process.
3. Types of sampling techniques
Probability sampling
I. Simple random
sampling
II. Stratified random
sampling
III. Systematic random
sampling
IV. Cluster sampling
V. Sequential sampling
Non probability
sampling
I. Purposive sampling
II. Convenient sampling
III. Consecutive sampling
IV. Quota sampling
V. Snow ball sampling
4. Probability sampling:
It involves random selection of elements. Every
subject in population has equal chance to be selected
as study sample.
5. I. Simple random sampling
Every population member has a equal chance of
being selected as a subject.
Prerequisites and uses:
Population should be homogenous
Must have list of elements/members of accessible
population
Random selection either by lottery, random table
or computer
6. Advantages
Most reliable and unbiased method
Requires minimum knowledge of study population
Free from sampling errors/bias.
Disadvantages
Expensive and time consuming.
7. II. Stratified random sampling
Dividing heterogeneous population in strata based
on selected traits such as age, gender, habitat and
then Random selection of sample from each Strata.
Prerequisites and uses:
Used for heterogeneous population
Advantages:
Ensures representative sample in heterogeneous
population
Comparison is possible in two groups.
8. Disadvantages:
Requires complete information of population
Large population is required
Chances of faulty classification of strata.
9. III. Systematic random sampling
Selecting of every Kth case from the group ,such as
every 10th person on a patient list or every 100th
person.
Prerequisites and uses
List of target population must be available
Random distribution subjects rather than
segregation.
10. Advantages
Convenient and simple to carry out
Distribution of sample over entire population.
Disadvantages
Less representative sample if subjects are non randomly
distributed.
11. IV. Cluster sampling
When simple random sampling is not possible due to
the size of the population, cluster random sampling
is carried out. Cluster sampling means random
selection of sampling unit consisting of population
elements.
Prerequisites and uses
When population is a very large
Random selection of subjects from selected clusters
Advantages
Cheap,quick and easy for a large population.
12. Disadvantages
Possibility of high sampling error
If a small homogenous population is under study,
this technique is not at all useful.
13. V. Sequential sampling
This method of sample selection is slightly different from
other methods. Here the sample size is not fixed. The
investigator initially selects small sample and tries out to
make inferences; if not able to draw results, he or she
then adds more subjects until clear-cut inferences can be
drawn.
14. Advantages
Facilitate the references for study on best possible
smallest sample.
Disadvantages
Not possible to study a phenomenon, which needs to be
studied one point of time.
Requires the repeated entry into the field to collect the
sample.
15. Non probability sampling technique wherein
the samples are gathered in a process that does not
give all individuals in the population with equal
chances of being selected in the sample. Elements
are chosen by choice not by a chance.
16. I. Purposive sampling
Subjects are chosen to be part of the sample with
specific purpose in mind. It is also known as
‘Judgmental’ or ‘Authoritative’ sampling. Samples are
chosen by choice not by chance.
Prerequisites and uses
Requires in-depth knowledge about the accessible
population
Used when a limited number of individuals a possess
the trait of interest.
17. Advantages
Simple to draw a sample
Saves resources as it requires less field work .
Disadvantages
Requires considerable knowledge about the population
under study
It is not always reliable sample.
18. II. Convenience sampling
Subjects are selected because of their convenient
accessibility and proximity to the researcher.
The subjects are choose just because of the fact that
there are very easy to recruit for the study. Sometimes, it
is also known as an ‘accidental sampling’.
19. Advantages
Easiest, cheapest and least time consuming
Helps in saving time, money and resources.
Disadvantages
Chances of Sampling bias
Findings can’t be generalized.
20. III. Consecutive sampling
It is very similar to convenience sampling, except
that it seeks to include all accessible subjects as a
part of the sample. This non probability sampling
technique can be considered as the best of all non
probability samples because it includes all the
subjects that are available. It is also known as ‘total
enumerative’ sampling.
21. Advantages
Ensures most representative sample
Convenient and less time consuming .
Disadvantages
The researcher has no set plans about sample size
and sampling schedule.
22. IV. Quota sampling
Equal or proportionate representation of subjects
from each quota. The bases of the quota are usually
age, gender, education, race, religion and social
economic status.
23. Advantages
Economically cheap economic
Suitable for studies where fieldwork has to be
carried out.
Disadvantages
Always does not guarantee representative sample
Chances of Sampling bias.
24. V. Snowball sampling
It is used by researchers to identify potential subjects
in studies where subjects are hard to locate. such as
commercial sex workers, drug abusers etc.
This type of Sampling technique work like chain
referral. Therefore it is also known as ‘chain
referral’ sampling. The researcher asks for
assistance from the subject to identify people with
similar trait of interest after observing the initial
subject .
Asking subjects to nominate another person with the
same trait.
25. Types of snowball sampling
1. Linear snowball sampling
2. Exponential non discriminative snowball sampling
3. Exponential discriminative snowball sampling.
26. Advantages
The process is simple , cheap and cost effective
The sampling technique requires a lesser workforce.
Disadvantages
Little control of researcher over the sampling
method
Representativeness of the sample is not guaranteed
Chances of poor coverage of entire population.