A presentation explaining Non - Probabilistic Sampling Method and it's various types with examples.

1. NON-PROBABILISTIC
SAMPLING
Abby Varghese
2nd M.Sc. Geography
Department of Geography
School of Earth Sciences

2. WHAT IS NON-PROBABILISTIC
SAMPLING?
Any sampling method where some elements of population have no
chance of selection (these are sometimes referred to as 'out of
coverage'/'under-covered'), or where the probability of selection can't
be accurately determined.
It involves the selection of elements based on assumptions regarding
the population of interest, which forms the criteria for selection.
Hence, because the selection of elements is non-random, non-
probability sampling does not allows the estimation of sampling
errors.

3. METHODS
•Convenience Samples (Ease of access)
•Snowball Sampling (Friend of Friend, etc.)
•Purposive sampling (Judgemental)
•Quota sampling

4. ISSUES
•Probability of being chosen is unknown
•Cheaper- but unable to generalize
•Potential for bias

5. CONVENIENCE SAMPLES
Convenience sampling is a non-probability sampling technique where
subjects are selected because of their convenient accessibility and
proximity to the researcher.

6. CONVENIENCE SAMPLES
The subjects are selected just because they are easiest to recruit for
the study and the researcher did not consider selecting subjects that
are representative of the entire population.
In all forms of research, it would be ideal to test the entire
population, but in most cases, the population is just too large that it
is impossible to include every individual.

7. USES
In pilot studies, convenience sample is usually used because it allows
the researcher to obtain basic data and trends regarding his study
without the complications of using a randomized sample.
This sampling technique is also useful in documenting that a
particular quality of a substance or phenomenon occurs within a
given sample. Such studies are also very useful for detecting
relationships among different phenomena.

8. CRITICISMS
Convenience sampling is sampling bias and that the sample is not
representative of the entire population. This may be the biggest
disadvantage when using a convenience sample because it leads to
more problems and criticisms.
Systematic bias stems from sampling bias. This refers to a constant
difference between the results from the sample and the theoretical
results from the entire population. A consequence of having
systematic bias is obtaining skewed results.
Convenience sample is the limitation in generalization and inference
making about the entire population. Since the sample is not
representative of the population, the results of the study cannot
speak for the entire population. This results to a low external
validity of the study.

9. SNOWBALL SAMPLING
It is also known as Chain Referral Sampling.
Snowball sampling is a non-probability sampling technique that is
used by researchers to identify potential subjects in studies where
subjects are hard to locate.

10. SNOWBALL SAMPLING
The process of snowball sampling is much like asking your subjects
to nominate another person with the same trait as your next subject.
The researcher then observes the nominated subjects and continues
in the same way until the obtaining sufficient number of subjects.

11. TYPES OF SNOWBALL
SAMPLING
•Linear Snowball Sampling
•Exponential Non-Discriminative Snowball Sampling
•Exponential Discriminative Snowball Sampling

12. ADVANTAGES OF SNOWBALL
SAMPLING
The chain referral process allows the researcher to reach populations
that are difficult to sample when using other sampling methods.
The process is cheap, simple and cost-efficient.
This sampling technique needs little planning and fewer workforce
compared to other sampling techniques.

13. DISADVANTAGES OF SNOWBALL
SAMPLING
The researcher has little control over the sampling method. The
subjects that the researcher can obtain rely mainly on the previous
subjects that were observed.
Representativeness of the sample is not guaranteed. The researcher
has no idea of the true distribution of the population and of the
sample.
Sampling bias is also a fear of researchers when using this sampling
technique. Initial subjects tend to nominate people that they know
well. Because of this, it is highly possible that the subjects share the
same traits and characteristics, thus, it is possible that the sample
that the researcher will obtain is only a small subgroup of the entire
population.

14. PURPOSIVE SAMPLING
Purposive sampling is a non-probability sampling technique where
the researcher selects units to be sampled based on their knowledge
and professional judgment.
This type of sampling technique is also known as Judgmental
sampling and authoritative sampling.

15. WHEN TO USE JUDGMENTAL
SAMPLING?
Judgmental sampling design is usually used when a limited number of
individuals possess the trait of interest. It is the only viable sampling
technique in obtaining information from a very specific group of
people. It is also possible to use judgmental sampling if the
researcher knows a reliable professional or authority that he thinks is
capable of assembling a representative sample.

16. DEMERITS
The two main weaknesses of authoritative sampling are with the
authority and in the sampling process; both of which pertains to
the reliability and the bias that accompanies the sampling technique.
Unfortunately, there is usually no way to evaluate the reliability of the
expert or the authority. The best way to avoid sampling error brought
by the expert is to choose the best and most experienced authority in
the field of interest.
When it comes to the sampling process, it is usually biased since
no randomization was used in obtaining the sample. It is also worth
noting that the members of the population did not have equal
chances of being selected. The consequence of this is the
misrepresentation of the entire population which will then
limit generalizations of the results of the study.

17. QUOTA SAMPLES
Quota sampling is a non-probability sampling technique wherein the
assembled sample has the same proportions of individuals as the
entire population with respect to known characteristics, traits or
focused phenomenon.

18. STEP-BY-STEP QUOTA SAMPLING
The first step in non-probability quota sampling is to divide the
population into exclusive subgroups.
Then, the researcher must identify the proportions of these
subgroups in the population; this same proportion will be applied in
the sampling process.
Finally, the researcher selects subjects from the various subgroups
while taking into consideration the proportions noted in the previous
step.
The final step ensures that the sample is representative of the entire
population. It also allows the researcher to study traits and
characteristics that are noted for each subgroup.

19. WHEN TO USE QUOTA SAMPLES
The main reason why researchers choose quota samples is that it
allows the researchers to sample a subgroup that is of great interest
to the study. If a study aims to investigate a trait or a characteristic of
a certain subgroup, this type of sampling is the ideal technique.
Quota sampling also allows the researchers to observe relationships
between subgroups. In some studies, traits of a certain subgroup
interact with other traits of another subgroup. In such cases, it is also
necessary for the researcher to use this type of sampling technique.

20. DISADVANTAGES OF QUOTA
SAMPLES
It may appear that this type of sampling technique is totally
representative of the population. In some cases it is not. Keep in
mind that only the selected traits of the population were taken into
account in forming the subgroups.
In the process of sampling these subgroups, other traits in the
sample may be overrepresented. In a study that considers gender,
socioeconomic status and religion as the basis of the subgroups, the
final sample may have skewed representation of age, race,
educational attainment, marital status and a lot more.