determining sampling methods cei 2015.0 implementing the reproductive health assessment toolkit for...

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