determining sampling methods cei 2015.0 implementing the reproductive health assessment toolkit for...
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Determining Sampling MethodsDetermining Sampling Methods
CEI 2015.0 Implementing the Reproductive Health Assessment Toolkit for Conflict-Affected Women
November 5, 2006
ObjectivesObjectives
At end of session, you will be able to:
• Understand importance of sampling
• Describe advantages/disadvantages of random and cluster sampling
• Obtain hands on sampling experience
What is Sampling?What is Sampling?
The procedure by which some members of a population are selected as representative of the entire population
Importance of SamplingImportance of Sampling
• Good sampling = representative data!!– Make reasonable inferences
• Decreases costs
• Speeds up data collection
• Balance feasibility
and precision
Toolkit SamplingToolkit Sampling
• Sampling unit = households (HH)• One woman of reproductive age (WRA)
per HH
• Choose random or cluster sampling
• Modify based on needs
AreaArea
AreaArea
Geographic BoundsGeographic Bounds
• Well-defined– Refugee or IDP camp– Groups of camps – Villages or towns where displaced people
live
• Use a map
Random SamplingRandom Sampling
Advantage Disadvantage
Random • More precise• Smaller sample
size
• Complete HH listing• Could use more
resources • More travel/time
Cluster SamplingCluster Sampling
Advantage Disadvantage
Cluster • No/incomplete
HH listing• Could use less
resources • Less travel
• Less precise• Larger sample size
Random vs ClusterRandom vs Cluster
Advantage Disadvantage
Random • More precise• Smaller sample
size
• Complete HH listing• Could use more
resources • More travel/time
Cluster • No/incomplete
HH listing• Could use less
resources • Less travel
• Less precise• Larger sample size
Random SamplingRandom Sampling
• Assumptions:– Prevalence rate of 50%– 95% confidence intervals +/- 5%– Response rate of 80%
• 400 completed interviews must sample 500 HH
Random Sampling StepsRandom Sampling Steps
• Step 1. Obtain HH lists
• Step 2. Identify HHs and select sample HHs to be surveyed
• Step 3. Select one WRA with each selected HH to be surveyed
Sample Household ListSample Household List
ID# Name Age Family size
# WRA Date of arrival
2416 X 31 6 2 11/27/2001
2723 X 26 4 1 8/22/1999
2714 X 43 4 1 4/21/2001
2448 X 53 5 0 7/1/2000
2578 X 28 4 2 11/20/2000
2463 X 34 5 1 7/5/1998
Household removed before
selection because no WRA
Practice Exercise: RandomPractice Exercise: Random
Random: Table 1Random: Table 1
Camp Households Household proportion
Sample size
Camp 1 367
Camp 2 405
Camp 3 245
Camp 4 271
Camp 5 350
Total 1638
367/1638 =22.4%
Random: Table 1Random: Table 1
Camp Households Household proportion
Sample size
Camp 1 367 367/1638 = 22.4%
Camp 2 405 405/1638 = 24.7%
Camp 3 245 245/1638 = 15.0%
Camp 4 271 271/1638 = 16.5%
Camp 5 350 350/1638 = 21.4%
Total 1638 100.0%
.224 x 500= 112
Random: Table 1Random: Table 1
Camp Households Household proportion
Sample size
Camp 1 367 367/1638 = 22.4% .224 x 500= 112
Camp 2 405 405/1638 = 24.7% .247 x 500= 124
Camp 3 245 245/1638 = 15.0% .150 x 500= 75
Camp 4 271 271/1638 = 16.5% .165 x 500= 83
Camp 5 350 350/1638 = 21.4% .214 x 500= 107
Total 1638 100.0% 501
Random Numbers TableRandom Numbers Table
• OR use computer applications
• For both random and cluster
• Example: Randomly select 112 HHs from 367 HHs for Camp 1.– How many digits -> 001-367, need 3 digits– Choose a direction -> decide to go right– Choose starting point – Read the number of digits– Repeat until 112 HHs selcted
Random Numbers TableRandom Numbers Table
A B C D E
1 8450 6992 6563 0340 2649
2 5952 1443 7100 8444 3904
3 5711 6779 9388 9668 4167
4 2681 8047 0494 7853 8411
5 0739 3114 3997 3482 3226
6 8985 2463 5054 3448 6357
437 (discard not in range)
Random Numbers TableRandom Numbers Table
A B C D E
1 8450 6992 6563 0340 2649
2 5952 1443 7100 8444 3904
3 5711 6779 9388 9668 4167
4 2681 8047 0494 7853 8411
5 0739 3114 3997 3482 3226
6 8985 2463 5054 3448 6357
100 HH is selected
for sample
Cluster SamplingCluster Sampling
• Can not guarantee precision
• Assumptions:– Response rate of 80%– At least 25 clusters with 25 HH in each
• Probability proportional to size (PPS)
• 500 completed surveys so sample 625 HHs
Cluster SamplingCluster Sampling• Step 1. Define clusters within geographic
boundaries• Step 2. Determine the number of HHs
within each cluster• Step 3. Select the clusters to be surveyed • Step 4. Select the HHs within each cluster
to be surveyed • Step 5. Select one WRA within each
selected HH to be surveyed
Practice Exercise: ClusterPractice Exercise: Cluster
Cluster: Step 3 Cluster: Step 3 Cluster HH in cluster Cumulative # of
HHsRange
1 26 26 1-26
2 30 56 27-56
3 27 83 57-83
4 50 133 84-133
5 31 164 134-164
6 28 192 165-192
7 35 227 193-227
8 (etc. . . ) 42 269 228-269
Cluster: Step 3Cluster: Step 3
• Sampling interval = cumulative # of HH divided by 25 clusters– Ex: 970/25 clusters = 39 (sampling interval)
• Randomly select number within sampling interval range– Ex: 01-39 = 31 is starting number
• Find where this number falls within range and cluster selected
Cluster: Step 3, select 1st cluster Cluster: Step 3, select 1st cluster Cluster HH in cluster Cumulative # of
HHsRange
1 28 28 1-28
2 32 60 29-60
3 65 125 61-125
4 48 173 126-173
5 25 198 174-198
6 29 227 199-227
7 37 264 228-264
8 (etc. . . ) 34 298 265-298
31 falls within range
Cluster 2 selected for
sample
Cluster: Step 3Cluster: Step 3
• Add sampling interval to first number: – Ex: 31 + 39 = 70
• Find where this number falls within range and cluster selected
• Continue until 25 clusters selected
Cluster: Step 3, select 2nd clusterCluster: Step 3, select 2nd clusterCluster HH in cluster Cumulative # of
HHsRange
1 28 28 1-28
2 32 60 29-60
3 65 125 61-125
4 48 173 126-173
5 25 198 174-198
6 29 227 199-227
7 37 264 228-264
8 (etc. . . ) 34 298 265-298
70 falls within range
Cluster 3 selected for
sample
Cluster: Step 3, select 3rd cluster Cluster: Step 3, select 3rd cluster Cluster HH in cluster Cumulative # of
HHsRange
1 28 28 1-28
2 32 60 29-60
3 65 125 61-125
4 48 173 126-173
5 25 198 174-198
6 29 227 199-227
7 37 264 228-264
8 (etc. . . ) 34 298 265-298
70 + 39=109 falls within
range
Cluster 3 selected again
for sample
Random vs ClusterRandom vs Cluster
Advantage Disadvantage
Random • More precise• Smaller sample
size
• Complete HH listing• Could use more
resources • More travel/time
Cluster • No/incomplete
HH listing• Could use less
resources • Less travel
• Less precise• Larger sample size
DRH Technical AssistanceDRH Technical Assistance
• Review sampling plan
• Consult on different sampling methods
www.cdc.gov/reproductivehealth/Refugee
Reproductive Health Assessment Toolkit for Conflict-Affected WomenReproductive Health Assessment
Toolkit for Conflict-Affected Women