Download Sampling to Study Drug Use

Sampling to Study
Drug Use
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Sampling to Study Drug Use:
Objectives
• Describe the principles on which
sampling is based
• Identify and describe different sampling
methods
• Select a sample of health facilities
from a list using at least two different methods
• Compare 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
judgments 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 that is sampled: for example, a person,
clinical episode, or health facility
• Study Population
- All the sampling units that could possibly 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:
- Nonprobability sampling
- Probability sampling
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Nonprobability 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 subsample 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
• Enumerators 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 1
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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