Notes for Data Users

1996 Data Limitations

For data year 1996, 31 states used the Mitofsky-Waksberg method of sampling, 14 states used a disproportionate stratified sample (DSS) design, and 6 states used a variety of other designs. The BRFSS standard for sample designs is that the sample must be justifiable as a probability sample of all households with telephones in the state. All but Alaska, California, Nevada and Texas met this criterion in 1996. The deviations from the standard by these states may reduce the comparability of their data with those of other areas.

Alaska used a stratified directory/random sample combination where the sample design is made up of 3 random digit-dialed strata and 4 listed strata. Telephone numbers in the listed strata are selected from a list of putative household telephone numbers. Forty-five percent of the completed interviews come from the listed strata. Alaska's sampling frame for the listed strata contains an estimated 50% of all households in these strata.

California used a truncated list-assisted design with sampling from 3+ blocks only. Noncoverage was further affected to an unknown degree by substitution for sample numbers used by any client of the commercial sample provider in the preceding six months. Probabilities of selection varied in proportion to the number of listed household telephone numbers in the 100 blocks associated with a number, the number of known business telephone numbers preceding a number, the number of preceding numbers selected by any client of the commercial sample provider in the preceding six months, and the measure of size used for each county, which could vary each time a sample was drawn.

Nevada used a truncated list-assisted design with sampling from 5+ blocks only. Noncoverage was further affected to an unknown degree by substitution for sample numbers used by any client of the commercial sample provider in the preceding six months. Probabilities of selection varied in proportion to the number of listed household telephone numbers in the 100 blocks associated with a number, the number of known business telephone numbers preceding a number, the number of preceding telephone numbers selected by any client of the commercial sample provider in the preceding six months, and the measure of size used for each county, which could vary each time a sample was drawn.

In addition to departing from the standard for sample designs, California modified the wording of mammography and Papanicolaou (PAP) test, and is therefore excluded from those tables in the report. California also asked the HIV/AIDS section questions to persons 18-44 years of age rather than those 18-64 years of age, and is therefore excluded from tables reporting data on this topic.

Although states collect data monthly, the number of interviews ranged from 1096 in New Mexico to 4003 in California in 1996. Estimates for some states are therefore based upon smaller sample sizes, and for rare events (e.g., diabetes), may yield unstable estimates.

Note: All data collection systems are subject to error, and records may be incomplete or contain inaccurate information. People may not remember essential information, a question may not mean the same thing to different respondents, and some individuals may not respond at all. It is not always possible to measure the magnitude of these errors or their impact on the data. The user must make his or her own evaluation on the data. Overall estimates generally have relatively small sampling errors, but estimates for certain population subgroups may be based on small numbers and have relatively large sampling errors. When the number of events is small and the probability of such an event is small, considerable caution must be observed in interpreting the conditions described by the figures.

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This page last reviewed June 22, 2005

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Centers for Disease Control and Prevention
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