Sample Provision Policy
Requesting Change in Sample Size During a Data Collection Year
A project wishing to obtain additional sample during a quarter should submit an e-mail request to Luann Rhodes (firstname.lastname@example.org) at least three weeks prior to the month in which additional sample is needed (e.g., May 10 for June 1 sample). A project wishing to increase or decrease sample size at the start of a quarter should submit an e-mail request to Luann Rhodes at least one month prior to start of the new quarter. (e.g., June 1 for third quarter sample). (Prior quarter's sample amount will be e-mailed to a projectís BRFSS coordinator for review one month prior to the start of the new quarter.)
Provide the following information by stratum to support your request:
1. Sample Record Request-
2. Prior Month(s) History by Stratum-
3. Identify any other factors that may have influenced number of completes.
E-mail call history files to Peter Mariolis, (email@example.com)
Review of Sample Requests
BSB will review requests as they are received and follow through with the BSB sample provider to ensure timely receipt of sample.
The review of a request to increase sample size will involve obtaining answers to at least the questions below. Other questions may also be addressed depending on the situation of an individual project. Additional questions may also be addressed as BSB gains more experience with requests for additional sample records and expanded quality control procedures.
The intention of BSB is to approve all requests from projects for which the answers are "Yes" to the first two sets of questions and "No" to the last two sets. Other answers imply the existence of substantial numbers of additional completes in the current sample and thus the possibility of an extreme amount of bias in the data and the non-comparability of the data to those of other BRFSS projects. In such cases, BSB may ask a project to make appropriate changes in its practices before approving a request for additional sample records.
For states which do not have any of the problems identified above, BSB will estimate the number of sample records needed, by stratum, based on the following formula:
Total number of replicates needed per month = Target monthly number of completes divided by Expected number of completes per replicate, where
Expected number of completes per replicate = [The average number of listed household numbers in previously generated replicates (for up a year previously) times the completion rate among listed household numbers (for up a year previously)] plus [the average number of not-listed, to-be-called, one-plus block numbers in previously generated replicates (for up a year previously) times the completion rate among not-listed, to-be-called, one-plus block numbers (for up a year previously)].
The resulting Total number of replicates needed per month will be routinely rounded up to the next whole number. If the fractional part of the initial calculation is greater than .75, the Total number of replicates will be rounded up an additional whole number. An additional adjustment may be made in agreement with the project under special circumstances.
For example, suppose a project has two geographic strata. The total number of replicates needed for each stratum has to be calculated separately. Suppose that the replicates of 50 contain an average of 11.2 listed household numbers and that the completion rate among the listed household numbers is .55. Suppose further that the replicates in the stratum contain an average of 10.5 not-listed, to-be-called, one-plus-block numbers and that the completion rate among these numbers is .30. The expected number of completes per replicate would be (11.2 x .55) + (10.5 x .30) = 9.31. Next, suppose that the monthly target number of completes in the stratum is 75. Then, the total number of replicates needed per month for that stratum would be 75 / 9.31 = 8.06, which would be rounded up to 9. The number of replicates needed per month for the second stratum would be calculated separately in the same way.
This formula ignores zero-block numbers because they result in so few completes. It takes into account previous project experiences with sample records and with outcome rates. BSB recommends that projects receiving sample from BSB in the previous year and not experiencing any of the problems indicated above use this formula to calculate their annual sample requests.
For states with extremely low completion rates, a rate higher than the observed rate might be substituted.
Purchase of Additional Sample Records by Projects, Including Contractors, With Their Own Funds
Projects or their contractors wishing to purchase additional sample
records from BSBís sample provider may do so if (1) they meet the criteria
specified above for requests for additional sample, including agreement of
BSB, (2) the sample records contain the same fields in the same format as
sample purchased through BSB, and (3) they make arrangements for BSB to
receive the additional sample records. BSB will process data records from
sample purchased directly by projects only if the SOS Section Chief or the
BSB Branch Chief has agreed to do so in writing beforehand and they meet
these three criteria.
This page last reviewed April 20, 2012[00_pgfooter2.htm]