BRFSS Weighting Documentation

Behavioral Risk Factor Surveillance System
Weighting BRFSS Data
BRFSS 2015

Weighting BRFSS Data
Introduction: Weighting Rationale
When data are used without weights, each record counts the same as any other record—and this implies
the following assumptions
1) Each record has equal probabilities of being selected.
2) Noncoverage and nonresponse are equal among all segments of the population.

During data collection, the conditions that actually happen may be different than the preceding
assumptions, and the differences can become large enough to affect the results of the data set. Weighting
each record appropriately can help to adjust for these differences (or assumption violations). Another
reason for weighting—one that is conceptually unrelated to the first reason—is to make the total number
of cases equal to some desired number. This state BRFSS data is the number of people in the state who
are aged 18 years and older. In the BRFSS, such raking serves as a blanket adjustment for noncoverage
and nonresponse and forces the total number of cases to equal population estimates for each geographic
region. For the BRFSS, this sums to the state population.
This document provides a general description of the process that reflects factors taken into
account in weighting the 2015 BRFSS data. Where a factor does not apply, its value is set to one for
calculation. Regardless of state sample design, use of the final weight in analysis is necessary if users
are to make generalizations from the sample to the population.

Weighting Methods and the BRFSS
From 1984 to 2010, the BRFSS used a technique called post-stratification to weight BRFSS survey data
to account for known proportions of age, race and ethnicity, gender, geographic region, and other known

characteristics of a population. This type of weighting was important because it helped the sample
represent the population accurately and adjusted for nonresponse bias. In 2011 a new statistical method
called raking replaced the older method because it could help record additional population
characteristics such as education level, marital status, and home ownership status of respondents. It also
helps the data fit into an analysis-friendly format called dual-frame survey design, so organizers could
account for the type of telephones the respondents used while completing the survey. Because of the
changes in the methodology, researchers are advised to avoid comparing data collected before the
changes (up to 2010) with data collected from 2011 and afterward. Details of these methodology
changes can be found with the BRFSS 2011 annual data page.

More About Raking
The raking weighting methodology is comprised of two sections—design weight and raking.
Design Weight = STRWT * (1/NUMPHON2) * NUMADULT
In 2014, the inclusion of cellular telephone respondents who also have landline telephones in their
household required an adjustment to the design weights to account for the overlapping sample frames.
From each of the two sample frames, the BRFSS calculated a compositing factor for the cellular
telephone respondents. This factor also indicated they had a landline telephone number, and—for the
landline telephone respondents—also indicated they had a cellular telephone number. The BRFSS did
this calculation by multiplying the design weight by the compositing factor to generate a composite
weight for the records in the overlapping sample frames. The BRFSS then truncated the design weight
based on quartiles within the geographic region and that processors used as the raking input weight.

The stratum weight accounts for differences in the basic probability of selection among strata (subsets of
area code or prefix combinations). It is the inverse of the sampling fraction of each stratum. There is
rarely a complete correspondence between strata and regions. The BRFSS defines strata by subsets of
area code or prefix combinations. The BRFSS defines regions by the boundaries of government entities.

The stratum weight (STRWT) is calculated using the following items

•
•
•

Number of available records (NRECSTR) and the number of records selected (NRECSEL)
within each geographic strata and density strata.
Geographic strata (GEOSTR), which may be the entire state or a geographic subset (e.g.,
counties, census tracts).
Density strata (_DENSTR) indicating the density of the phone numbers for a given block
of numbers as listed or not listed.

Within each _GEOSTR*_DENSTR combination, the stratum weight (_STRWT) is calculated from the
average of the NRECSTR and the sum of all sample records used to produce the NRECSEL. The stratum
weight is equal to NRECSTR/NRECSEL.

1/ NUMPHON2

The inverse of the number of residential telephone numbers in the
respondent’s household.

NUMADULT

The number of adults 18 years and older in the respondent’s household.

FINAL WEIGHT

The BRFSS rakes the design weight to 8 margins (age group by gender,
race or ethnicity, education, marital status, tenure, gender by race or
ethnicity, age group by race or ethnicity, phone ownership). If BRFSS
includes geographic regions, four additional margins (region, region by
age group, region by gender, region by race or ethnicity) are included. If at
least one county has 500 or more respondents, BRFSS includes four
additional margins (county, county by age group, county by gender,
county by race or ethnicity).

_LLCPWT

The final weight assigned to each respondent.

The BRFSS uses weight trimming to increase the value of extremely low weights and decrease the value
of extremely high weights. The objective of weight trimming is to reduce errors in the outcome
estimates caused by unusually high or low weights in some categories.

The population estimates obtained for building the target totals are from similar sources used in previous
years. Intercensal population estimates were purchased from The Nielsen Company, LLC, at the countylevel for age, race or ethnicity, and gender. These population estimates are used as the population totals
for a state across all margins. The 5-year American Community Survey PUMS dataset (2010-2014) was
used to obtain estimates for margins 3, 4, and 5 (education, marital status, tenure). The
noninstitutionalized adults were weighted by the person-level weights to generate the population
estimates. The percentages were then used in the raking margins. The telephone ownership estimates for

margin 8 were taken from the state wireless estimate percentages produced by NCHS and released in
February, 2016.

Calculation of a Child Weight
The BRFSS calculates the design weight for the child weighting from the stratum weight times the
inverse of the number of telephones in the household and then multiplies it by the number of children
Child Design Weight = STRWT * (1/NUMPHON2) * CHILDREN
CHIILDWT =

BRFSS rakes the child design weight to 5 margins—age by gender, race or
ethnicity, gender by race or ethnicity, age by race or ethnicity, and phone ownership.
_CLLCPWT is the weight assigned for each child interview.

BRFSS Weighting prior to 2011
Please note prior to 2011, the data weighting formula used post-stratification as the weighting method.
The following formulas apply to data the BRFSS weighted prior to 2011.
FINALWT = STRWT * (1/ NUMPHON2) * NUMADULT * POSTSTRAT

The BRFSS intended the preceding computational formula to reflect all the possible factors that it could
take into account in weighting a state’s data. Where a factor does not apply, the BRFSS set its value to
one.
FINALWT is

the final weight BRFSS assigns to each respondent.

The stratum weight accounts for differences in the basic probability of selection among strata (subsets of
area code or prefix combinations). It is the inverse of the sampling fraction of each stratum. There is
rarely a complete correspondence between strata and regions. The BRFSS defines strata by subsets of
area code or prefix combinations. Regions are defined by the boundaries of government entities. The
BRFSS calculates the stratum weight (STRWT) using the following items
•
•

Number of available records (NRECSTR) and the number of records selected (NRECSEL)
within each geographic strata and density strata.
Geographic strata (GEOSTR) may be the entire state or a geographic subset (e.g.,
counties, census tracts).

•

Density strata (_DENSTR) indicates the density of the phone numbers for a given block of
numbers as listed or not listed.

Within each _GEOSTR*_DENSTR combination, the BRFSS calculates the stratum weight (_STRWT) from
the average of the NRECSTR and the sum of all sample records used to produce the NRECSEL. The
stratum weight is equal to NRECSTR/NRECSEL.
1/ NUMPHON2

The inverse of the number of residential telephone numbers in the respondent’s
household.

NUMADULT

The number of adults 18 years and older in the respondent’s household.

POSTSTRAT is

the number of people in an age-by-gender or age-by-race-by-gender category in
the population of a region or a state divided by the sum of the products of the preceding weights
for the respondents in that same age-by-gender or age-by-race-by-gender category. It adjusts for
noncoverage and nonresponse and, before 1995, also adjusted for different probabilities of
selection by region, where applicable.
Calculation of a Child Weight prior to 2011
CHILDWT = STRWT * (1/ NUMPHON2) * CHILDREN * POSTCH

The BRFSS intends the preceding computational formula to reflect all the possible factors that could be
taken into account in weighting a state’s data. Where a factor does not apply, BRFSS sets its value to
one.
CHILDWT is

the final weight the BRFSS assigns to each randomly selected child as identified by the
adult respondent.
BRFSS calculates the design weight for the child weighting from the stratum weight times the inverse of
the number of telephones in the household and then multiplies it by the number of children
Child Design Weight = STRWT * (1/ NUMPHON2) * CHILDREN
POSTCH is the number of children in an age-by-gender or age-by-race-by-gender category in the
population of a region or a state divided by the sum of the products of the preceding weights for
the children in that same age-by-gender or age-by-race-by-gender category. It adjusts for
noncoverage and nonresponse.

Calculation of a Household Weight prior to 2011
HOUSEWT = STRWT * (1/ NUMPHON2) * POSTHH

HOUSEWT is

the weight BRFSS assigns to each household.

The BRFSS calculates the design weight for the household weighting from the stratum weight times the
inverse of the number of telephones in the household
Household Design Weight = STRWT * (1/NUMPHON2)
POSTHH is

the number of households in the population of a region or a state divided by the sum
of the products of the preceding weights for the households in that same category. It adjusts for
noncoverage and nonresponse.