Prevention. Promotion. Protection.

Methodology

The Behavioral Risk Factor Surveillance System (BRFSS) is conducted by the Alaska Division of Public Health in cooperation with the National Centers for Disease Control and Prevention (CDC). It is a monthly telephone survey that utilizes a standard protocol and interviewing methods developed by the CDC.

Sample Design

Although the main purpose of the BRFSS is to estimate the prevalence of behavioral risk factors in the general population, interviewing each person is not economically feasible. Thus, a probability (or random) sample is selected in which all persons have a known chance of selection. The BRFSS in Alaska uses a stratified random sampling design. The Alaska sample is stratified into five regions based on common demographics. An equal number of interviews are conducted from each region, which purposely oversamples the nonurban areas of Alaska.

Sample Size

Each month over 200 Alaska residents age 18 and older are interviewed over the telephone regarding their health practices and day to day living habits, to reach an annual sample size of 2,500 (500 per region). The data are collected from January through December, for each year.

Sampling Process

From 1990 - 1998, the telephone sample was generated by the University of Alaska Anchorage, Institute of Social and Economic Research (ISER). The method preferred by the CDC for generating a random sample of telephone numbers works efficiently when telephone prefixes are heavily saturated with working residential telephone numbers. Since most of the prefixes in Alaska have less than 500 residential numbers, the probability of reaching a working residential number is low. For this reason, the Institute of Social and Economic Research used a combination method of computer random generation (using the RANDY method) for large exchanges and random selection from a database of entered directory numbers for small exchanges. In 1997, this strategy was re-evaluated and in 1998 ISER modified its methods to include more random numbers from small exchanges. In addition, GENESYS ID services were purchased each month for the generated sample, in order to eliminate as many business and non-working lines as possible.

Beginning in 1999, and presumably for the future, the BRFSS began to rely on CDC for its telephone sample purchased from GENESYS. This aimed to improve and calculate the probability that all households in Alaska with telephones would have a chance of inclusion in the study. The sample currently used is a Disproportionate Stratified Sample Design (DSS). Disproportionate stratified random sampling is a variation of cluster sampling. For DSS, information obtained from other sources is used to classify 100 number blocks of telephone numbers into two strata based on the presumed density (high or low) of residential telephone numbers (strata that are either likely or unlikely to yield residential numbers). Telephone numbers in the "likely" strata are sampled at a higher rate than numbers in the unlikely strata.The GENESYS sample is divided into zero banks and one-plus banks. These values are determined by analyzing all possible 100 blocks for an area. The recommended sampling ratio between one-plus blocks and zero blocks is 4:1. Since the rural region of Alaska has as many as 80% of phone banks that are zero blocks, the sampling ratio is 8:1 in Region 4. This ratio was determined in consultation with BSB. In 2003, zero blocks were dropped from the random sample.

Because Alaska has such a low number of active residential lines, Alaska requires a large amount of phone sample each month to operate successfully. In addition, GENESYS is electronically identifying business and non working numbers through its ID services and has modified its ID services to detect non working numbers in rural Alaska through its Super ID services which has made technological adjustments to improve the process and increase the survey efficiency for Alaska.

Survey Instrument

The BRFSS instrument is a standardized questionnaire which consists of three sections:

  • the core (which includes demographics),
  • a set of optional modules and
  • state specific questions.

The questionnaire covers such topics as Health Status, Health Care Access, Nutrition, Physical Activity, Diabetes, Tobacco Use (including Smokeless Tobacco), Alcohol Use, Demographics, Women's Health, Injury Prevention, and HIV/AIDS Awareness.

Participation is random, anonymous and confidential. Respondents are randomly selected from among the adult members of the household. Only those living in households are surveyed. Those living in institutions (i.e., nursing homes, dormitories) are not surveyed.

Data Collection

Interviews are conducted by trained college interns and administrative clerks. The interviews are conducted everyday including evenings and weekends.

Data are collected via computer using Ci3 CATI (Computer Assisted Telephone Interviewing) software. Monthly data files are sent to the Centers for Disease Control and Prevention.

Data Analysis

The Behavioral Risk Factor Surveillance System (BRFSS) data contains information on Alaskan adults only (aged 18 and above).

Data collected by BRFSS are edited using PCEdits software produced by the CDC. Edit reports are produced monthly and corrections made. Corrected data files and edit reports are sent to the CDC monthly. At the end of each survey year, data are compiled and weighted by CDC, and cross tabulations and prevalence reports are prepared.

Weighting:

Unweighted data are the actual responses of each survey respondent. The data are weighted or adjusted to compensate for the overrepresentation or under-representation of persons in various subgroups. The data are further weighted to adjust the distribution of the sample data so that it reflects the total population of the sampled area.

Data Reporting:

Data are analyzed by the CDC for Alaska by gender, race, age, marital status, education, income and employment and standard tables are produced.

Special Note:

For 2000 and 2001, health care coverage results for Alaska are further analyzed by the Alaska Division of Public Health, Bureau of Vital Statistics. This analysis adjusts for survey respondents who report they have no health care coverage and then in a follow up question report to be covered by a health care plan. This explains the reason that prevalence estimates may not match those published by the CDC.

Comparisons

All prevalence comparisons made to the National BRFSS Ranges and the National BRFSS Median are comparisons made to the 50 states participating in the Behavioral Risk Factor Surveillance System.

Limitations

The BRFSS uses telephone interviewing for several reasons. Telephone interviews are faster and less expensive than face to face interviews. Calls are made from one central location (Juneau) and are monitored for quality control.

The one main limitation of any telephone survey is that those people without phones cannot be reached and are not represented. In Alaska, about 97% of households have phones; about 98% of all U.S. households have phones (2000 US Census, Summary File 4). The percentage of households with a telephone varies by region in Alaska . In general, persons of low socioeconomic status are less likely than persons of higher socioeconomic status to have phones and are undersampled. However, national BRFSS results correspond well with findings from other surveys conducted in person.

With surveys based on self-reported information, the potential for bias must be kept in mind when interpreting results. Survey response rates may also affect the potential for bias in the data.

The reliability of a prevalence estimate depends on the actual, unweighted number of respondents in a category or demographic subgroup (not a weighted number). Interpreting and reporting weighted numbers that are based on a small, unweighted number of respondents can be misleading. The degree of precision increases if the sample size is larger and decreases if the sample size is smaller. Prevalence estimates are not usually reported for those categories in which there were less than 50 respondents and are rounded to the nearest whole percent when the denominator is less than 500.