BRFSS Methodology

Target Population

The BRFSS attempts interviews with adult Virginia residents (18 years of age or older) whose household telephone number is included in the sampling frame.

---

Selection Criterion

First, interviews must identify if the number they have dialed is a household. A household is eligible if it is a housing unit with a separate entrance where occupants eat separately from other persons on the property and is occupied by its members as their principal or secondary place of residence.

Noneligible households include the following:

• Vacation homes not occupied by household members for more than 30 days per year;
• Group homes (e.g. sororities and fraternities, halfway houses, shelters); and
• Institutions (e.g. nursing homes, college dormitories).

Once a telephone number is determined to be a household, a random selection procedure must be used to select the appropriate household member to interview.  

Household members include:

• All related adults 18 years old or older, unrelated adults, roomers and domestic workers who consider the household their home, even though they may not be home at the time of the call.
• Does not include adult family members who are currently living elsewhere, such as at college, a military base, a nursing home or a correctional facility.

A BRFSS respondent is 18 years of age and older.  The first question on the BRFSS questionnaire regarding household selection asks how many members in the household are 18 years of age or older:   

• If the answer is one and the person answering the telephone is that adult, the interviewer proceeds to the first question on the questionnaire.
• If the response indicates that there are five or more adults, the interviewer should probe to ensure that they all are 18 years of age or older, that all are currently living in the household and that the household is not a group home or institution.

The next question asks how many of the adults are men and how many are women. The interviewer enters the appropriate answers. If there is only one adult in the household, the interviewer enters "1" and leaves the other box blank. If there are more than one adults living in the household, the interviewer asks to speak to the adult with the most recent birthday.  This adult is then selected to complete the BRFSS.

Sampling Design

Sampling design refers to the method used to select respondents to interview.  This is also known as sampling method or sampling strategy.

The method of choosing telephone numbers must be statistically valid. The resulting sample must be a probability sample so that information obtained from the sample can be used to generalize results to the total population in the state as well as to the nation as a whole.  Samples used in the BRFSS must be probability samples in which all households with telephones have a known, nonzero chance of inclusion. 

With Disproportionate Stratified Random sampling (DSS), as implemented for the BRFSS beginning in 2003, telephone numbers are drawn from two strata (lists) that are based on the presumed density of known telephone household numbers. In this design, telephone numbers are classified into strata that are either high density (listed 1+ block telephone numbers) or medium density (not listed 1+ block telephone numbers) to yield residential telephone numbers. Telephone numbers in the high density stratum are sampled at the highest rate. The rate at which each stratum is sampled is called the sampling rate. The ratio of the sampling rate of one stratum to sampling rate of a reference stratum is called the sampling ratio.

The DSS design attempts to find a way of differentiating, before sampling begins, between a set of telephone numbers that contains a large proportion of target numbers (the high-density block) and a set that contains a smaller proportion of target numbers (the medium-density block). It is possible to create more than two groups, but for BRFSS, only two groups are used. In this way, sampling telephone numbers is more efficient compared to simple random sampling.

Sample Size

Since 2003, the goal of the Virginia BRFSS has been to complete at least 5,000 interviews with Virginia adults annually. 

Interview Year	Number of BRFSS Completions
1997	3,524
1998	3,506
1999	3,502
2000	1,996
2001	2,939
2002	4,392
2003	5,442
2004	5,576
2005	5,232
2006	5, 453
2007	6, 203

Cell Phone Sampling

Virginia was one of six states that participated in a cell phone sample pilot in 2008.  The goal was to complete at least 250 BRFSS interviews with respondents on their cell phones.  For participating and completing an interview, respondents were given a ten dollar ($10) amazon gift card. 

Cell Phone Methodology

If the number was incorrect or the person was less than 18 years of age, the interview was not conducted. If it was not a safe time to conduct the survey, the respondent was called back at a later date. In addition to about 10 minutes of health questions, a variety of questions about cell telephone usage and whether the respondent lived in a household with a landline—to establish if he or she was a dual-user (cell telephone and landline) or cell telephone-only user.

In previous studies, Macro found that approximately 90 to 95 percent of respondents reported living in the state for which the cell telephone sample was drawn. Furthermore, cell telephone-only respondents were more likely to respond than those who also had a landline. At the start of the pilot study, Macro hypothesized that evenings and weekends would be most productive since subscribers usually receive free minutes during these times. However, empirical evidence suggested just the opposite, and weekday interviewing was very productive. Macro's research suggests that most respondents have their cell telephones on all, or most of, the day and a large percentage are between the ages of 18 and 24 (a group whose participation rates have declined in contemporary RDD samples).  Macro will draw on the knowledge gained through the cell telephone pilot to collect high-quality data for this cell phone sampling frame project.

Respondent Eligibility

• Interviewers will determine if they have reached an eligible person by asking scripted screening questions that will be incorporated into the survey questionnaire.  Macro assumes that eligible respondents would include those who:
• Confirm that Macro dialed the correct telephone number;
• Live in a private household;
• Current resident of state from which their telephone number was selected;
• Confirm that we reached them on their cell telephone, not business only phone; and
• Are 18 years of age or older;

For 2009, the Virginia BRFSS will complete a minimum of 250 state-wide BRFSS interviews with cell phone only users.  It is mandatory for every state to complete a minimum of 250 completed BRFSS interviews and a maximum of 10% of their total BRFSS completions. 

Stratification

From 1997 to 1999, and again from 2002 through 2008, the Virginia Department of Health
(VDH) funded additional data collection to allow stratification of the state by the thirty-five (35) health districts.  Approximately 100 interviews were collected annually in each health district. Although a larger sample would have been ideal, use of the pooled data allows for comparisons to be drawn on key BRFSS indicators. 

The Virginia BRFSS work group and the BRFSS coordinator will be working closely together in the summer and fall of 2006 to evaluate, and possibly revise, the current sampling and stratification methodologies.  Virginia, similar to other states, has had a significant increase in minority populations in the state.  Due to this, a methodology that over samples certain minority populations would provide more accurate and representative health data on adults living in the state of Virginia.

In 2007 and 2008, the Virginia BRFSS over sampled the Hispanic population in Virginia.  The over sample concentrated on areas of the state that were identified as having high percentages of Hispanic adults.  These areas were concentrated in Northern Virginia, the Tidewater area and areas in Southwestern Virginia. 

Due to issues related to high cell phone usage and mobility among this population, the percentage of interviews successfully completed are very low.  Because of this, Virginia will not implement a Hispanic over sample in 2009 and will develop a plan on how to better represent this population for the 2010 BRFSS.

Links to the thirty-five (35) local health districts websites:

Weighting Adjustment

Each annual file is weighted to the estimated non-institutionalized adult population of Virginia that year in each of the 35 health districts.  BRFSS data are also directly weighted for the probability of selection of a telephone number, the number of adults in a household and the number of telephones in a household. A final poststratification adjustment is made for nonresponse and noncoverage of households without telephones, racial/ethnic origin, sex and age. The weights for each relevant factor are multiplied to get a final weight.

The weighting calculations are calculated by CDC staff separately for each of the 35 geographic strata (health districts). The weighting scheme is designed to allow each health district to stand on its own, as well as to obtain meaningful statewide prevalence estimates. The prevalence is the proportion of people in a population with a certain condition or health behavior. The data included in this report are statewide prevalence estimates of certain risk factors and health behaviors. Since prevalence provides the magnitude of disease in a population at given point in time, it is useful for planning and allocation of health resources. Periodic estimation of point prevalence is useful in tracking changes in disease patterns over time.

---

---