Community Medicine. sampling methods.

1. METHOD OF SAMPLING
• Sampling - the process of selecting a portion of the
total population.
• Population refers to the whole collection of units from
which a sample may be drawn. (Target population)
• Sample – subset of the population (Study
population).
3/2/2024 1

2. METHOD OF SAMPLING
• Any sample selected must be representative of the
population and should reflect the characteristics of
the population from which it is drawn.
• This allows the investigator to generalize the findings
to the wider population.
3/2/2024 2

3. METHOD OF SAMPLING
Two major groups:
1. Probability sampling method
• All units have a known probability of selection;
• Sample chosen is representative of the population
and results can be generalized to the wider
population.
3/2/2024 3

4. METHOD OF SAMPLING
(a) Simple random sampling method
• Each unit has an equal and known chance of being
selected from the total population.
• Techniques employed may include balloting, table of
random numbers, computer generated random
numbers.
• Useful for homogenous populations.
• Requires a sampling frame - a list of all units in the
population.
3/2/2024 4

5. METHOD OF SAMPLING
(b) Systematic sampling method
• Every nth unit is selected from a list of all units in the
population using a sampling interval (total
population/sample size).
• The 1st unit is selected from the first n units in the
population list using a simple random sampling
method.
• Useful for homogenous populations.
• Requires a sampling frame.
3/2/2024 5

6. METHOD OF SAMPLING
(c) Stratified sampling method
• First, divide the population into well-defined
subgroups (strata) according to some important
characteristics e.g. age, sex, etc.
• Sampling fraction (sample size divided by the total
population) is calculated and applied to each stratum
to get the required sample size.
3/2/2024 6

7. METHOD OF SAMPLING
• Thereafter, simple random sampling technique is used
to select participants using the same sampling
interval in each group.
• Using same sampling fraction for all strata ensures
proportionate representation in the sample.
• Useful for heterogenous population.
3/2/2024 7

8. METHOD OF SAMPLING
(d) Cluster sampling method
• The unit of selection (sampling unit) includes groups
rather than individuals.
• The population is divided into clusters.
• A probability sampling method usually a simple
random sampling is used to select one or more
clusters.
• All members of the selected cluster(s) is/are included
in the study.
• Useful for largely homogenous populations and
where no sampling frame exists.
3/2/2024 8

9. METHOD OF SAMPLING
(e) Multistage sampling method:
• Using a combination of two or more of the
aforementioned sampling methods in more than two
stages for recruiting study participants.
• Useful for heterogenous population.
3/2/2024 9

10. METHOD OF SAMPLING
(f) Multiphase sampling method:
• Information is collected from an initial sample drawn
and additional information or measurement taken
from a sub-sample of the initial sample.
• For example, in a Tb survey:
• Phase I - MT in all cases
• Phase II - CXR in MT positive cases
• Phase III - Sputum exam in CXR +ve cases
3/2/2024 10

11. METHOD OF SAMPLING
2. Non-probability sampling method
• No known chance of selection of all units;
• Sample is usually not representative of the population
from which it was drawn.
3/2/2024 11

12. METHOD OF SAMPLING
Convenient/Accidental sampling:
• As the name implies, the sample is selected because
they are available or easily accessible.
• The sample is biased because the researchers may
unconsciously select some kind of respondents and
avoid others.
• This method is used during preliminary or exploratory
research.
3/2/2024 12

13. METHOD OF SAMPLING
Purposive sampling
• The individuals in the sample are selected because
they possess one or more specific characteristics that
the investigator is interested in.
3/2/2024 13

14. METHOD OF SAMPLING
Quota sampling:
• Sample is selected non-randomly based on some
fixed quota.
• The population is first divided into sub-groups.
• Convenient or purposive sampling is used to select
the required number of subjects from each sub-
group.
3/2/2024 14

15. METHOD OF SAMPLING
Snowball sampling
• The initial subject or subjects who meet the criteria
for inclusion in the study are first identified.
• These in turn identify and recruit others who they
know meet the desired criteria.
• This method is useful for populations that are
inaccessible or hard to find.
3/2/2024 15