Weighting Your Data

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Weighting Your Data
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General Information

• Allows you to generalize your results to the
  entire population
• Necessary for all sampled based surveys
• Requirements
• Scientifically selected sample
• Complete documentation
• High overall response rate

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What does weighting do?

• It accounts for
• the probability of selection within the
  population
• non-response
• distribution of the target population by age and
  sex
• It allows one participant to represent many others

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Probability of Selection from the Sample
(Individual Weight)

• Accounts for the variation in the selection
  probabilities in the sample
• 1/(selection probability of cluster 1)
  (selection probability of cluster 2)(as many
  clusters until reach primary sampling unit)

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Example

• There are 12 districts and 3 are randomly
  selected probability of selection at district
  level .25

District Towns selected Towns in district Probability of selection (town) INDIVIDUAL WEIGHT
A 8 30 .27 1/(.25.27)
B 3 10 .30 1/(.25.30)
C 5 22 .23 1/(.25.23)
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Non-Response

• Non-response can cause bias
• Information on response must be collected during
  interviews
• Interview tracking form collects this information
• Entered during data entry
• Automatically attached to dataset with Epi Info
  program

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Calculating the Non-Response Weight

• Calculate non-response for each age and sex
  stratum
• Non-response weight
• 1/(response rate for age-sex stratum)

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Example
Males by age Response Rate (RR) Non-response weight 1/RR
2534 .94 1.064
3544 .95 1.053
4554 .87 1.149
5564 .79 1.266
Non-response weights would be calculated
for both men and women.
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Population Distribution

• Used to adjust the sample to the target
  population
• "Post-stratification adjustments"
• Need population information stratified by age and
  sex to calculate

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Calculating Population Weight

• Population weight
• Proportion of population/Proportion of sample
• Population weight
• (Age-Sex of population / Total population)
• (Age-Sex of sample / Total Sample)

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Example
Males by age Pop N Proportion of population (A) Sample N Proportion of sample (B) Weight A/B
2534 2,000 .200 30 .075 2.667
3544 1,760 .176 40 .100 1.760
4554 1,440 .144 50 .125 1.152
5564 1,600 .160 60 .150 1.067
Total 10,000 400
Post stratification would be calculated for
males and females
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Overall Weight for Individual Steps

• W1 Individual weight
• W2s1 Non-response weight
• W3 Population weight
• WStep1 W1W2s1W3
• WStep2 W1W2s2W3
• WStep3 W1W2s3W3

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Weighting and STEPS

• Tools to help calculate weights
• STEPSsampling.xls
• Interview_Tracking_Form.xls

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Weights

• The Weights spreadsheet is used to
• document the sample selection and
• attach the weights to the dataset in Epi Info
• Part of the spreadsheet is automated and parts
  need to be filled in by hand

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Weights Example
Information available from Rand Hhold spreadsheet
Weighting Info
Automatically entered
Enter by hand by matching information from PSU
and Clustering SSU
Information available from Clustering SSU
spreadsheet Weighting Info
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Attaching the Weights to Your Dataset

• Once you have documented your weights using the
• STEPSsampling.xls
• Interviewtracking.xls
• THEN
• You can use the generic analysis programs to
  automatically attach the weights to your dataset