Weighting Household Surveys By David F. Pearson, Ph.D., P.E. April 2007
Household Surveys in Texas Small sample Stratified by household size and income Telephone recruitment and data retrieval
Sampling Plan Samples not proportional to population household distribution Plan uses population household distribution and expected trip making Survey samples weighted for results to represent population
Household Weight Based on surveyed households and estimated population of households Computed for each cell in stratification matrix
Household Weight Example 4,297 is household population of Size 1 with annual income between 0 and $9, is number of households surveyed of Size 1 with annual income between 0 and $9, is weight given to each household surveyed 2003 Smith County Household Weight Factors
Use of Weights - Examples Estimate population Estimate number of trips by purpose Estimate TLFDs by trip purpose Estimate vehicle availability distribution
Use of Weights - Examples Estimate employment, including self- employed individuals Develop household trip rates for use in TDMs Estimate persons by age and sex, including person trip rates
Results Dilemma Population estimates did not agree with expected values Population estimates by age and sex did not agree with expected values Employed persons were significantly different than estimates from other sources Concern – Potential bias
Revised Approach Develop additional weight based on results from household weights Apply additional weight only to person variables
Revised Approach Steps Estimate number of persons by age cohort and gender using household weights Compute person factor based on population of persons by age cohort and gender
Calculating Person Weight Factor i-age cohort, e.g., 40-to-44 years of age j-gender, e.g., male or female PW-person weight factor POP-population, i.e., number of persons EST-estimate of number of persons from applying household factor
How It Works Household variables weighted using household factor Examples of these variables include: - Number of vehicles available - Household income - Number of bicycles available - Household length of residence
How It Works Person variables weighted using household AND person factor Examples of these variables include: - Person trips - Persons licensed to drive - Persons employed (including occupation and industry) occupation and industry) - Student status - Days worked at home
Area Characteristics
Distribution of Persons by Age
Person Trips per Person by Age
Distribution of Persons by Age
Person Trips per Person by Age
Comparison of Estimates Smith County
Comparison of Estimates Hidalgo County
Comparison of Trip Estimates Smith County
Comparison of Trip Estimates Hidalgo County
Employment Estimate Comparisons
Findings Use of two-stage weighting procedure – Improves estimates of HBW travel – Improves estimates of employed persons in area
Questions