Weighting Household Weighting Household SurveysSurveys

By David F. Pearson, Ph.D., P.E.By David F. Pearson, Ph.D., P.E.

April 2007April 2007

Household Surveys in TexasHousehold Surveys in Texas

Small sampleSmall sample

Stratified by household size and Stratified by household size and incomeincome

Telephone recruitment and data Telephone recruitment and data retrievalretrieval

Sampling PlanSampling Plan

Samples not proportional to Samples not proportional to population household distributionpopulation household distribution

Plan uses population household Plan uses population household distribution and expected trip makingdistribution and expected trip making

Survey samples weighted for results Survey samples weighted for results to represent populationto represent population

Household WeightHousehold Weight

Based on surveyed households and Based on surveyed households and estimated population of householdsestimated population of households

Computed for each cell in Computed for each cell in stratification matrixstratification matrix

Household Weight ExampleHousehold Weight Example

• 4,297 is household population of Size 1 with annual incomebetween 0 and $9,999

• 48 is number of households surveyed of Size 1 with annualincome between 0 and $9,999

• 89.52 is weight given to each household surveyed

2003 Smith County Household Weight FactorsHousehold Size

Household Income 1 2 3 4 +

$0 – $9,999 4297/48=89.52

$10,000 – $19,999 3253/58=56.09

$20,000 – $34,999 2305/26=88.65

$35,000 – $49,999

$50,000 Plus

Use of Weights - ExamplesUse of Weights - Examples

Estimate populationEstimate population

Estimate number of trips by purposeEstimate number of trips by purpose

Estimate TLFDs by trip purposeEstimate TLFDs by trip purpose

Estimate vehicle availability Estimate vehicle availability distributiondistribution

Use of Weights - ExamplesUse of Weights - Examples

Estimate employment, including self-Estimate employment, including self-employed individualsemployed individuals

Develop household trip rates for use Develop household trip rates for use in TDMsin TDMs

Estimate persons by age and sex, Estimate persons by age and sex, including person trip ratesincluding person trip rates

Results DilemmaResults Dilemma

Population estimates did not agree with Population estimates did not agree with expected valuesexpected values

Population estimates by age and sex did Population estimates by age and sex did not agree with expected valuesnot agree with expected values

Employed persons were significantly Employed persons were significantly different than estimates from other different than estimates from other sourcessources

Concern – Potential biasConcern – Potential bias

Revised ApproachRevised Approach

Develop additional weight based on Develop additional weight based on results from household weightsresults from household weights

Apply additional weight only to Apply additional weight only to person variablesperson variables

Revised Approach StepsRevised Approach Steps

Estimate number of persons by age Estimate number of persons by age cohort and gender using household cohort and gender using household weightsweights

Compute person factor based on Compute person factor based on population of persons by age cohort population of persons by age cohort and genderand gender

Calculating Person Weight FactorCalculating Person Weight Factor

i - age cohort, e.g., 40-to-44 years of agej - gender, e.g., male or femalePW - person weight factorPOP - population, i.e., number of personsEST - estimate of number of persons from applying

household factor

ij

ij

ij EST

POPPW

How It WorksHow It Works

Household variables weighted Household variables weighted using household factorusing household factor

Examples of these variables include:Examples of these variables include:- Number of vehicles available- Number of vehicles available- Household income- Household income- Number of bicycles available- Number of bicycles available- Household length of residence- Household length of residence

How It WorksHow It Works Person variables weighted using Person variables weighted using

household AND person factorhousehold AND person factor

Examples of these variables include:Examples of these variables include:

- Person trips- Person trips

- Persons licensed to drive- Persons licensed to drive

- Persons employed (including- Persons employed (including

occupation and industry)occupation and industry)

- Student status- Student status

- Days worked at home- Days worked at home

Area CharacteristicsArea Characteristics Smith County Hidalgo County

Year Surveyed 2003 2004 Households 69,650 181,473 Households Surveyed 1,079 1,321 Population 183,582 658,028 Median Income $37,500 $ 21,832 Employment 84,403 183,644

Distribution of Persons by AgeDistribution of Persons by Age

Smith County

0

1000

2000

3000

4000

5000

6000

7000

8000

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Cohort

Nu

mb

er o

f P

erso

ns

Male Population

Male Estimate

Distribution of Persons by AgeDistribution of Persons by Age

Smith County

0

1000

2000

3000

4000

5000

6000

7000

8000

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Cohort

Nu

mb

er o

f P

erso

ns

Female Population

Female Estimate

Person Trips per Person by AgePerson Trips per Person by Age

Smith County

0

1

2

3

4

5

6

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Cohort

Per

son

Tri

ps

Per

Per

son

Males

Females

Distribution of Persons by AgeDistribution of Persons by Age

Hidalgo County

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Cohort

Nu

mb

er o

f P

erso

ns Male Population

Male Estimate

Distribution of Persons by AgeDistribution of Persons by Age

Hidalgo County

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Colhort

Nu

mb

er o

f P

erso

ns

Female Population

Female Estimate

Person Trips per Person by AgePerson Trips per Person by Age

Hidalgo County

0

1

2

3

4

5

6

16-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80+

Age Cohort

Per

son

Tri

ps

Per

Per

son

MALES

FEMALES

Comparison of EstimatesComparison of EstimatesSmith CountySmith County

Household

Weight Only Household and Person Weight

Variable

Households 69,650 69,650 Population 173,019 183,590 Persons Employed 73,814 82,424

Comparison of EstimatesComparison of EstimatesHidalgo CountyHidalgo County

Household

Weight Only Household and Person Weight

Variable

Households 181,473 181,473 Population 634,373 658,028 Persons Employed 228,575 247,892

Comparison of Trip EstimatesComparison of Trip EstimatesSmith CountySmith County

Household

Weight Only

Household and Person

Weight

Percent Difference

Type

Home Based Work 87,751 97,117 10.6 %

Home Based Non-Work 339,074 342,481 1.0 %

Non-Home Based 223,806 233,158 4.2 %

All Purposes 650,631 672,756 3.4 %

Comparison of Trip EstimatesComparison of Trip EstimatesHidalgo CountyHidalgo County

Household

Weight Only

Household and Person

Weight

Percent Difference

Type

Home Based Work 336,388 363,367 8.0 %

Home Based Non-Work 1,283,779 1,334,360 3.9 %

Non-Home Based 551,296 576,112 4.5 %

All Purposes 2,171,463 2,273,839 4.7 %

Employment Estimate ComparisonsEmployment Estimate Comparisons

0

50,000

100,000

150,000

200,000

250,000

300,000

Smith Co. Hidalgo Co.

County

Em

plo

ymen

t E

stim

ates

TWC Employment Estimate

Employed Persons Estimate

FindingsFindings

Use of two-stage weighting Use of two-stage weighting procedureprocedure– Improves estimates of HBW travelImproves estimates of HBW travel– Improves estimates of employed Improves estimates of employed

persons in areapersons in area

QuestionsQuestions