2001 Behavioral Risk Factor Surveillance System (BRFSS) Metropolitan Area Data Weighting Methodology

Creation of Metropolitan-Level Weights

This description of the Behavioral Risk Factor Surveillance System (BRFSS) 2001 metropolitan area data is based on a report produced for CDC by RTI International.  

A documented and verified subset of the 2001 BRFSS has been produced to provide some local area estimates.  These local areas are identified as metropolitan statistical areas (MSAs), primary metropolitan statistical areas (PMSAs), or New England county metropolitan statistical areas (NECMAs), as defined by the U.S. Office of Management and Budget and used by the U.S. Census Bureau.  In this report, all mentions of metropolitan area refer to MSAs, PMSAs, and NECMAs.  Additional information on MSA, PMSA, and NECMA designations used by the U.S. Census Bureau is available at http://www.census.gov/population/www/estimates/aboutmetro.html and 
http://www.census.gov/population/www/estimates/pastmetro.html.  The metropolitan area definitions used for this analysis were those used by the Census Bureau in 2001, not those used currently.

The BRFSS 2001 metropolitan area data set was produced by adding new analysis weights designed to correspond to the 2001 population estimates for each eligible metropolitan area.  The additional weights were post-stratified to the metropolitan area level. The process by which these new weights were obtained is detailed in Appendix C, “Weight Class Collapsing Rules.”

Selected Areas 

Typically, BRFSS data are used to produce state-level estimates.  However, for this project, 2001 BRFSS data were used to produce prevalence estimates for metropolitan areas. Estimates have been produced for 69 metropolitan areas that have met the analysis criteria for the 2001 data year.   Missing county codes were imputed from a value included in the purchased telephone sample that represents the county most likely to be associated with the telephone number.  

Appendix A: Variables Added to 2001 Data

ANLPMSA—Indicates whether a respondent lived in a metropolitan area that was included in the re-weighting.

0—Not in a metropolitan area that was re-weighted 
1—In a metropolitan area that was re-weighted 

_PMSA—Metropolitan area code of the MSA, PMSA, or NECMA, if appropriate, in which the respondent lived.  

_PMSANAM—Metropolitan area name of the MSA, PMSA, or NECMA, if appropriate, in which the respondent lived.  

_PMSAWT—The new metropolitan area-level weight, which should be used when generating metropolitan area-level estimates for questions that were asked of the entire sample.  

_PMSWTSS—The new metropolitan area-level weight for the split-sample questions, which should be used when generating metropolitan area-level estimates for the split-sample questions.  This variable is missing for respondents who did not live in the Chicago, IL, PMSA.

PMSAFLAG—Indicates whether the metropolitan area in which the respondent lived was associated with additional weight class collapsing problems beyond those addressed by the collapsing rules.

1 = Metropolitan area that was post-stratified by sex and age only and had at least one weighting class group that failed the 80% rule (see Appendix C for an explanation of the weight class collapsing rules).  After adjacent age categories were collapsed when necessary, weighting classes with fewer than 19 cases remained.                                       
2 = Metropolitan area that was post-stratified by sex and age only and had at least one weighting class group that failed the 80% rule.  After adjacent age categories were collapsed when necessary, all weighting classes had at least 19 cases.                                                
3 = Metropolitan area that was post-stratified by sex and age only and had no weighting class group that failed the 80% rule.  After adjacent age categories were collapsed when necessary, weighting classes with fewer than 19 cases remained.                                             
4 = Metropolitan area that was post-stratified by race, sex, and age and had at least one weighting class group that failed the 80% rule.  The group(s) that were collapsed across race needed additional collapsing across age categories.  After adjacent age categories were collapsed when necessary, weighting classes with fewer than 19 cases remained.            
5 = Metropolitan area that was post-stratified by race, sex, and age and had at least one weighting class group that failed the 80% rule.  The group(s) that were collapsed across race also needed additional collapsing across age categories.  After adjacent age categories were collapsed when necessary, all weighting classes had at least 19 cases.                            
6 = Metropolitan area that was post-stratified by race, sex, and age and had at least one weighting class group that failed the 80% rule.  The group(s) that were collapsed across race needed no additional collapsing across age categories, but other groups that were not collapsed across race did need additional collapsing across age categories.  After adjacent age categories were collapsed when necessary, weighting classes with fewer than 19 cases remained.                                                               
7 = Metropolitan area that was post-stratified by race, sex, and age and had no weighting class group that failed the 80% rule.  After adjacent age categories were collapsed when necessary, weighting classes with fewer than 19 cases remained. 

Appendix B: The 69 Areas with Metropolitan Area-Level Weights in the 2001 BRFSS, with MSA, PMSA, or NECMA Codes and Names

0200	Albuquerque, NM MSA	
0520	Atlanta, GA MSA	
0720	Baltimore, MD PMSA	
0760	Baton Rouge, LA MSA	
0875	Bergen-Passaic, NJ PMSA	
1000	Birmingham, AL MSA	
1080	Boise City, ID MSA	
1123	Boston–Worcester–Lawrence–Lowell–Brockton, MA–NH NECMA	
1303	Burlington, VT NECMA	
1520	Charlotte–Gastonia–Rock Hill, NC–SC MSA	
1600	Chicago, IL PMSA	
1640	Cincinnati, OH–KY–IN PMSA	
1680	Cleveland–Lorain–Elyria, OH PMSA	
1840	Columbus, OH MSA	
1920	Dallas, TX PMSA	
2080	Denver, CO PMSA	
2120	Des Moines, IA MSA	
2160	Detroit, MI PMSA	
2190	Dover, DE MSA	
2520	Fargo–Moorhead, ND–MN MSA	
3120	Greensboro–Winston-Salem–High Point, NC MSA	
3160	Greenville–Spartanburg–Anderson, SC MSA	
3283	Hartford, CT NECMA	
3320	Honolulu, HI MSA	
3360	Houston, TX PMSA	
3400	Huntington–Ashland, WV–KY–OH MSA	
3480	Indianapolis, IN MSA	
3640	Jersey City, NJ PMSA	
3760	Kansas City, MO–KS MSA	
4120	Las Vegas, NV–AZ MSA	
4360	Lincoln, NE MSA	
4400	Little Rock–North Little Rock, AR MSA	
4480	Los Angeles–Long Beach, CA PMSA	
4520	Louisville, KY–IN MSA	
4920	Memphis, TN–AR–MS MSA	
5000	Miami, FL PMSA	
5015	Middlesex–Somerset–Hunterdon, NJ PMSA	
5080	Milwaukee–Waukesha, WI PMSA	
5120	Minneapolis–St Paul, MN–WI MSA	
5190	Monmouth–Ocean, NJ PMSA	
5360	Nashville, TN MSA	
5380	Nassau–Suffolk, NY PMSA	
5483	New Haven–Bridgeport–Stamford–Waterbury–Danbury, CT NECMA	
5523	New London–Norwich, CT NECMA	
5560	New Orleans, LA MSA	
5600	New York, NY PMSA	
5640	Newark, NJ PMSA	
5720	Norfolk–Virginia Beach–Newport News, VA–NC MSA	
5880	Oklahoma City, OK MSA	
5920	Omaha, NE–IA MSA	
6160	Philadelphia, PA–NJ PMSA	
6200	Phoenix–Mesa, AZ MSA	
6280	Pittsburgh, PA MSA	
6440	Portland–Vancouver, OR–WA PMSA	
6483	Providence–Warwick–Pawtucket, RI NECMA	
6640	Raleigh–Durham–Chapel Hill, NC MSA	
6660	Rapid City, SD MSA	
6720	Reno, NV MSA	
7040	St Louis, MO–IL MSA	
7160	Salt Lake City–Ogden, UT MSA	
7600	Seattle–Bellevue–Everett, WA PMSA	
7760	Sioux Falls, SD MSA	
8003	Springfield, MA NECMA	
8280	Tampa–St Petersburg–Clearwater, FL MSA	
8520	Tucson, AZ MSA	
8560	Tulsa, OK MSA	
8840	Washington, DC–MD–VA–WV PMSA	
9040	Wichita, KS MSA	
9160	Wilmington–Newark, DE–MD PMSA	

Appendix C:  Weight Class Collapsing Rules

Metropolitan Area-Level Weighting Methodology

Respondents were assigned to a metropolitan area on the basis of the county name that they provided during the interview.  The county was then recorded using the Federal Information Processing Standards (FIPS) county code used by the U.S. Census Bureau.  Respondents with unknown or missing values for the county code were excluded.

All respondents in metropolitan areas were then assigned to age, race, and sex categories.  If a respondent’s age was missing, it was imputed using the variable _IMPAGE available in the BRFSS public-use 2001 data file.  If a respondent’s race was missing, it was imputed using the majority race for the metropolitan area in which the respondent lived.  

Within each metropolitan area, respondents were assigned to weighting classes on the basis of age (18-24, 25-34, 35-44, 45-54, 55-64, or 65+), race (white and non-Hispanic, or nonwhite or Hispanic), and sex (male or female) categories.  Some states did not use race in post-stratification; for the metropolitan areas in these states, only age and sex groups were used to establish weighting classes.  For all other metropolitan areas, all three groups were used to establish weighting classes.  For metropolitan areas that crossed state lines, the post-stratification variables used by the state in which the majority of the metropolitan area’s population lived were used to set up weighting classes.  Thus, metropolitan areas that post-stratified by race had 24 initial weighting classes and metropolitan areas that did not post-stratify by race had 12 initial weighting classes.  

Weighting classes with fewer than 19 sample members were collapsed according to the following rules:

1.	For metropolitan areas that used race in post-stratification, the race categories within a sex category were collapsed if at least 80% of the age categories (i.e., at least five age categories) in the race and sex cross-classification had fewer than 19 members. In metropolitan areas that used race to create the initial weighting classes, the number of weighting classes was thus reduced from 24 to 12 if race was collapsed for both sexes and from 24 to 18 if race was collapsed for only one sex.

2.	The two youngest age categories in any age-sex or age-sex-race weighting class were collapsed if either contained fewer than 19 members.  The two middle and two oldest age categories in each remaining weighting class were also collapsed if either had fewer than 19 members. 

3.	Weighting classes were not collapsed across sex.

4.	Metropolitan area was not included in the re-weighting if weighting classes with fewer than 19 sample members remained after all collapsing rules had been applied. 

In 69 metropolitan areas, at least 500 respondents participated in the 2001 BRFSS and at least 19 sample members remained in all final weighting classes.  A list of these metropolitan areas is available in Appendix B.  The respondents in these metropolitan areas were the only ones assigned a metropolitan area-level weight.  To calculate the new metropolitan area-level weight, we applied a post-stratification adjustment factor to the design weight (_WT2) and created the adjustment factor based on the ratio of the total population to the sum of the design weights for each weighting class within each metropolitan area.  The new metropolitan area-level weight (_PMSAWT) should be used to generate estimates in these 69 metropolitan areas.

Example—SUDAAN Code

For example, suppose we want a prevalence estimate for the Atlanta, GA metropolitan area  (PMSA code = 0520).  The SAS/SUDAAN code that could be used to generate this estimate is:

proc sort data=xxxx;
by _STSTR _PSU;
run;

proc descript data=xxxx filetype=sas design=wr;
nest _STSTR _PSU / missunit;
weight _PMSAWT;
subpopn _PMSA=0520 / name=” Atlanta, GA”;
var (your analysis variable);
catlevel (the level of your analysis variable for which you want an estimate);
run;

Metropolitan Area-Level Split-Sample Weighting Methodology

In 2001, the state of Illinois used a split sample; the state’s sample was divided in half and a different version of the questionnaire was administered to each half.  The _PMSAWT is appropriate for analyzing the responses to questions asked on both versions of the Illinois questionnaire.  An additional weight was created to use with questions that were asked on only one version of their questionnaire.  The metropolitan area-level split-sample weight (_PMSWTSS) was created using the methodology described above, except that respondents in the Illinois metropolitan areas were separated according to the questionnaire version they received.  Weighting classes were created for each questionnaire version within each metropolitan area.  Adjustment factors were applied to the design weight (_WT2) that forced the sum of the weights for each half of the sample to sum to population totals.  The Chicago, IL metropolitan area was the only metropolitan area with adequate split-sample sizes to provide metropolitan area-level split-sample weights.  

Example—SUDAAN Code:

For example, suppose we want a prevalence estimate for a split-sample question from questionnaire version 1 (_QSTVER) for the Chicago, IL metropolitan area (PMSA code = 1600).  The SAS/SUDAAN code that could be used to generate this estimate is:

Data xxxx;
Set yyyy;

If (_PMSA=1600 & _QSTVER=1) then DUMMY = 1;

Run;

proc sort data=xxxx;
by _STSTR _PSU;
run;

proc descript data=xxxx filetype=sas design=wr;
nest _STSTR _PSU / missunit;
weight _PMSWTSS;
subpopn DUMMY=1 / name=” Chicago, IL – questionnaire version 1”;
var (your analysis variable);
catlevel (the level of your analysis variable for which you want an estimate);
run;

Note:  In general, estimates for split-sample questions can only be generated for metropolitan areas that do not cross state lines.  For example, the St. Louis, MO-IL metropolitan area lies in both Illinois and Missouri.  These new metropolitan area-level weights cannot be used to generate a prevalence estimate for a question that was asked of only half of the Illinois residents but all of the Missouri residents in the sample.  For metropolitan areas that cross state boundaries, estimates can only be generated for questions asked of the entire sample population in each state.