Weighting Your Data

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

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

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)]

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)

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

Calculating the Non-Response Weight Calculate non-response for each age and sex stratum Non-response weight = 1/(response rate for age-sex stratum)

would be calculated for Example Males by age Response Rate (RR) Non-response weight = 1/RR 25–34 .94 1.064 35–44 .95 1.053 45–54 .87 1.149 55–64 .79 1.266 Non-response weights would be calculated for both men and women.

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

Calculating Population Weight Proportion of population/Proportion of sample (Age-Sex of population / Total population) (Age-Sex of sample / Total Sample)

Example Males by age Pop N Proportion of population (A) Sample N Proportion of sample (B) Weight = A/B 25–34 2,000 .200 30 .075 2.667 35–44 1,760 .176 40 .100 1.760 45–54 1,440 .144 50 .125 1.152 55–64 1,600 .160 60 .150 1.067 Total 10,000 400 Post stratification would be calculated for males and females

Overall Weight for Individual Steps W1: Individual weight W2s1: Non-response weight W3: Population weight WStep1= W1*W2s1*W3 WStep2= W1*W2s2*W3 WStep3= W1*W2s3*W3

Weighting and STEPS Tools to help calculate weights STEPSsampling.xls Interview_Tracking_Form.xls

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

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

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