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Technical Information and Data
Numbered Memos
BRFSS POLICY MEMO 2003.1
Sampling and Sampling Information Protocols
The protocols below supersede those found in BRFSS Policy Memo 98.3 and
99.1.
- Beginning with the 2003 data year, zero block numbers will not be
included in the sample of telephone numbers drawn from the Telecordia
database. The sampling frame will consist of all telephone numbers in
one-plus blocks. In the Telecordia database, every prefix in the United
States and its dependencies is assigned an NXX type number which
designates the kind of service that is provided through the prefix. The
NXX Types that contain household telephone numbers are
00 Regular (Plain Old Telephone Service, or POTS)
50 Shared Among Three or More Services
51 Shared Between POTS and Mobile
52 Shared Between POTS and Paging
54 Shared Between POTS and Cellular
Thus, every one-plus block telephone number in the NXX Types 00, 50, 51,
52, and 54 should have a non-zero probability of being selected.
- Beginning with the 2003 data year, this item will be standard BRFSS
policy.
A disproportionate stratified sample (DSS) design is defined in this
document as a design in which telephone numbers are assigned to two strata
based on the presumed density of telephone household numbers. The ideal
sampling ratio between listed and not listed one-plus block density
stratum, that which yields the smallest standard error for a fixed cost or
a given sample size, equals the square root of the ratio of the cost of
completing an interview in a high (listed) and medium (not-listed)
household density stratum.
- The ideal sampling ratio between listed one-plus block and
not-listed one-plus block household density strata in a DSS design
equals the square root of the ratio of the cost of completing an
interview in the listed one-plus block stratum compared to the
not-listed one-plus block stratum. In the absence of data on the
relative cost of completing an interview in the two strata, states
should use a sampling ratio of 1.5:1 in which the listed will be sampled
at the rate of 1.5 times that of not-listed. States wanting to use a
larger ratio must submit a request to do so to BSB, with documentation
that the actual cost of obtaining a completed interview from medium
density strata is greater than 4 times the cost of obtaining a completed
interview from higher density strata. This request must be agreed upon
in writing by the SOS Section Chief or the BSB Branch Chief.
- States using definitions for high and medium density strata other
than listed one-plus blocks and not-listed one-plus blocks must submit a
proposed sampling ratio to BSB based on actual or estimated cost ratios.
The sampling ratio should approximately equal the square root of the
ratio of the cost of completing an interview in a high and medium
household density stratum. This sampling ratio must be agreed upon in
writing by the Survey Operations Team Leader Chief or the BSB Branch
Chief prior to its use.
- The telephone numbers used to conduct a survey consist of all numbers
actually called by the data collector plus, if applicable, those numbers
pre-identified as non-working or business by the sample provider which
were not called by the data collector but would have been called had they
not been so identified.
For data year 2003, all states are strongly encouraged to follow the
provisions of this item. The monthly data files sent to BSB must consist
of a record for every telephone number used to conduct the BRFSS survey,
including all numbers pre-identified as non-working or business which were
not called but would have been called had they not been so identified.
- Commercial sample providers usually have many fields of information
available about the sample telephone records that they generate. The data
collection process generates many additional pieces of information for
each sample record related to the data collection effort. BSB is committed
to making as many of these fields available on data records as is
practical given constraints of time and money.
Beginning with data year 2003, every state must, as applicable, include
the following fields in each complete or incomplete data record sent to
BSB. These items are used for quality assurance studies, data weighting,
and analyses of sample quality. Results from the analyses will be shared
with the states through individual reports and as part of the
documentation of the annual file. Specifications for these fields are
contained in the BRFSS Item By Item Coding Instructions and Editing
Criteria.
- The Number of Sample Records Selected from a Stratum by the Sample
Provider
- The Number of Telephone Numbers (Eligible Sampling Units) in the
Stratum from Which the Sample Was Selected
- Annual Sequence Number
- Replicate Number
- Replicate Depth
- Number of Attempts
- Precalling Identification Status (1=Neither non-working nor
business, 3=Non-working, 5=Business)
- Density Strata (1=Listed household number, 2=Not a listed household
number)
- Any deviations from the above protocols must be agreed upon in writing
by the Survey Operations Team Leader or the BSB Branch Chief.
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