Download What is Sampling?

Sampling for Drug Use
Indicators Study
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Sampling for Drug Use
Indicators Study: Objectives
• Describe the principles upon which
sampling is based.
• Identify and describe different sampling
methods.
• Select a sample of health facilities
from a list of such facilities using
at least two different methods.
• Compare the the results of the methods.
• Instruct others how to sample for
drug use studies.
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What is Sampling?
• Sampling is a
process by which
we study a small
part of a population
to make judgements
about the entire
population.
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Sampling involves selecting a number of
units from a defined population.
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Sampling Definitions
• Sampling Unit
- The thing which is sampled, for example, a person,
a clinical episode, or a health facility
• Study Population
- All the sampling units which could possible be
included in the sample
• Sampling Frame
- A list of all the available sampling units in the study
population
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A Representative Sample
A representative sample has all the
important characteristics of the
study population from which it is
drawn.
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Sampling Methods
• Two categories of sampling
methods:
- Non probability sampling
- Probability sampling
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Non Probability Sampling
Methods
• Convenience Sampling
- study units available at
the time of data collection
are selected for the sample
• Quota Sampling
- different categories of
sample units are included
until a certain number has
been reached in each
category
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Probability Sampling Methods
•
•
•
•
•
Simple Random Sampling
Systematic Sampling
Stratified Sampling
Cluster Sampling
Multistage Sampling
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Simple Random Sampling
• Used in situations where
the number of sampling
units is relatively small
• Process:
- Identify all possible units
available for sampling
- Decide on the size of the
sample
- Choose units by a lottery
method
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Systematic Sampling With
Equal Probability
• Numbered list of all possible units.
• # units  desired sample size = sampling interval.
– For example, to select 20 health centers from a list of 46, the
sampling interval is 46/20 = 2.3
• Random # x sampling interval = random start.
– For example, if the random number is 0.183 calculate 0.183 × 2.3
= 0.421, which rounds upward to 1
• Round number up to choose sample unit.
• Add sampling interval to random start for subsequent units.
– For example,
0.421 + 2.3 = 2.721 or Facility 3
2.721 + 2.3 = 5.021 or Facility 6
5.021 + 2.3 = 7.321 or Facility 8 and so forth.
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Systematic Sampling With
Probability Proportional To Size
• List where the units are sorted in decreasing order
by some measure of size (like population or number
of visits)
• Calculate the cumulative total.
• Cumulative total  sample size = sampling interval.
• Random # x sampling interval = random start.
• Choose first unit with cumulative total result.
• Add sampling interval to previous total for
subsequent units.
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Stratified Sampling
• Used when the sampling frame contains clearly
different categories (strata).
–For example:
• Urban and rural facilities
• Facilities with and without doctors
• Government and mission facilities
• Process:
- Organize the list of sampling units by stratum
- Select units within each stratum using a random
method (simple random sampling or systematic
sampling)
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Cluster Sampling
• Used when for logistic reasons it is easier to select
sample units in groups
• Process
- Select a cluster of sample units– Example: health center with multiple prescribers
• Include the entire cluster or select a sub- sample
– OR
- Select a random sample unit to start each cluster (a
house, a patient, etc.)
- Include neighboring sample units until a certain cluster
size is reached
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Multistage Sampling
• Randomly select primary sampling units at the first
stage:
– specific communities
– specific health facilities
• Within the primary sampling units, randomly select the
final sampling units at the second stage:
– drug use encounters
– patients
– households
• Sometimes in complex samples, additional stages are
needed
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Sample Size
• The optimal sample size is often a compromise
between what is statistically Desirable and
what is practically Feasible.
In general, a minimum
sample size is 30.
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Sample Size
Confidence Intervals
If the real figure is 50%
• Sample size
50
100
200
500
1000
• Range of Confidence
36 - 64%
40 - 60%
43 - 57 %
45 - 55 %
47 - 53 %
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Conclusion
• Principles of sampling should be
understood by everyone involved in drug
use studies.
• Eumerators and supervisors should
understand that any facility, any
prescription, or any patient should have a
chance to be included.
• Make every effort to avoid bias in selecting
units for study.
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Activity One
Sample Selection
In this exercise, groups will use
the different sampling methods
on a real data set to select a
sample. The results of the
different sampling methods will
be compared.
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