Non-probability sampling in social science.

1. Prepared by;
Abdul Shafwan T H
2nd M.A IR&PS
Central University of Kerala

2. Sampling
 Measuring a small portion of something and then making
a general statement about the whole thing.
 Process of selecting a number of units for a study in such
a way that the units represent the larger group from which
they are selected.

3. Why We Need Sampling?
 Sampling makes possible the study of a large, (different
characteristics) population.
 Sampling is for economy
 Sampling is for speed.
 Sampling is for accuracy.
 Sampling saves the sources of data from being all
consumed.

4. General Types of Sampling
1. Probability sampling
2. Non-probability sampling

5. Non-probability sampling
 Unequal chance of being included in the sample (non-random)
 Non random or non - probability sampling refers to the sampling process
in which, the samples are selected for a specific purpose with a pre-determined
basis of selection.
 The sample is not a proportion of the population and there is no system in
selecting the sample. The selection depends upon the situation.
 No assurance is given that each item has a chance of being included as a sample
 There is an assumption that there is an even distribution of characteristics within
the population, believing that any sample would be representative.

6. Types of Non-Probability Sampling
 Judgment or purposive or deliberate sampling
 Convenience sampling
 Quota sampling
 Snow Ball Sampling

7. 1. Judgment or purposive or deliberate
sampling
 In this method, the sample selection is purely based on the
judgment of the investigator or the researcher. This is
because, the researcher may lack information regarding the
population from which he has to collect the sample.
Population characteristics or qualities may not be known,
but sample has to be selected.
 In this method of sampling the choice of sample items
depends primarily on the judgment of the researcher. In
other words, the researcher determines and includes those
items in the sample which he thinks are most typical of the
universe with regard to the characteristics of research
project.

8.  For example, suppose 100 boys are to be selected from
a college with 1000 boys. If nothing is known about
the students in this college, then the investigator may
visit the college and choose the first 100 boys he meets.
Or he may select 100 boys all belonging to III Year. Or
he might select 25 boys from Commerce course, 25
from Science courses, 25 boys from Arts courses and 25
from Fine arts courses. Hence, when only the sample
size is known, the investigator uses his discretion and
select the sample.

9. The use of judgment sampling is justified by following premises:
 If there are a small number of sampling units is in the universe,
judgment sampling enables inclusion of important units.
 Judgment stratification of population helps in obtaining a more
representative sample in case research study wants to look into
unknown traits of the population.
 Judgment sampling is a practical method to arrive at some solution to
everyday business problems.
Limitations:
 The judgment sampling involves the risk that the researcher may
establish conclusions by including those items in the sample which
conform to his preconceived ideas.
 There is no objective way of evaluating the reliability of sample results.

10. 2. Convenience sampling
 Convenience sampling is commonly known as unsystematic, accidental
or opportunistic sampling. According to this procedure a sample is
selected according to the convenience of the investigator.
 In this method of sampling the choice of sample items depends
primarily on the judgment of the researcher. In other words, the
researcher determines and includes those items in the sample which he
thinks are most typical of the universe with regard to the
characteristics of research project.
 A type of non probability sampling which involves the sample being
drawn from that part of the population which is close to hand. That is,
readily available and convenient.
 For example, suppose 100 car owners are to be selected. Then we may
collect from the RTO's office the list of car owners and then make a
selection of 100 from that to form the sample.

11. A convenience sampling may be used in the following
cases:
 i) When universe is not well defined,
 ii) When sampling unit is not clear, and
 iii) When complete list of the source is not available.

13. 3. QUOTA SAMPLING
 In this method, the sample size is determined first and then quota is
fixed for various categories of population, which is followed while
selecting the sample.
 In this method the quota has to be determined in advance and
intimated to the investigator. The quota for each segment of the
population may be fixed at random or with a specific basis. Normally
such a sampling method does not ensure representativeness of the
population.
 Example: - Suppose we want to select 100 students, then we might say
that the sample should be according to the quota given below : Boys
50%, Girls 50% Then among the boys, 20% college students, 40% plus
two students, 30% high school students and 10% elementary school
students. A different or the same quota may be fixed for the girls.

14. MERITS OF QUOTA SAMPLING
 Reduces cost of preparing sample and field work, since ultimate
units can be selected so that they are close together.
 Introduces some stratification effect.
DEMERITS OF QUOTA SAMPLING
 Introduces bias of investigator is not involved at any stage, the
errors of the method cannot be estimated by statistical
procedures.
 Since random sampling is not involved at any stage, the errors of
the method cannot be estimated by statistical procedures. Quota
sampling is most commonly used in marketing survey and
election polls.

15. 4. SNOWBALL SAMPLING
 It refers to Identifying someone who meets the criteria
for inclusion in the study.
 Selection of additional respondents is based on
referrals from the initial respondents.