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What are we covering and what we are not?

Centers for Disease Control and Prevention (CDC) Introduces the changes in its methodology. Presented by: Khaleel S. Hussaini PhD Bureau Chief, Public Health Statistics Division of Public Health Preparedness Judy Bass Arizona’s BRFSS Coordinator June 26, 2012.

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What are we covering and what we are not?

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  1. Centers for Disease Control and Prevention (CDC) Introduces the changes in its methodology.Presented by:Khaleel S. Hussaini PhDBureau Chief, Public Health StatisticsDivision of Public Health PreparednessJudy BassArizona’s BRFSS Coordinator June 26, 2012

  2. What are we covering and what we are not? • We are talking about BRFSS and changes in methods. • What are the changes and why? • Implications of the changes. • We are not talking about questions and modules, but will schedule a separate time and place. • We will not discuss costs? • We will not discuss time-lines for the next report.

  3. So what are the changes? Any guesses! BRFSS weighting process

  4. The Behavioral Risk Factor Surveillance Survey (BRFSS) was initiated by CDC since 1984. It is the largest ongoing telephone survey in the World. Like all surveillance systems BRFSS has adapted to both technical and non-technical changes. Technical terms to remember POPULATION VS. SAMPLE

  5. Population inferences can be made...

  6. ...by selecting a representative sample from the population

  7. SAMPLING STRATEGY IN BRFSS

  8. Disproportionate stratified sample

  9. National Center for Health Statistics Estimates of Cell Phone Only Households in the United States, 2008-2011 Source: Blumberg SJ, Luke JV. Wireless substitution: Early release from the Nation Health Interview Survey, January2008 -June 2011 Available at: http://www.cdc.gov/nchs/data/nhis/earlyrelease /wireless201112.pdf. Accessed June 14, 2012.

  10. According to CDC Cell phone interviewers can now be better represented as the sample is: • More likely to be younger; • More renters than home owners; • More likely to be Hispanic; • More likely to be single; • There are also differences in attitude and behaviors between cell phone only users and those with landline phones.

  11. Why weight? Weighting procedures

  12. What is weighting? • Sampling weights are needed to correct for imperfections in the sample that might lead to bias. It can include the selection of units with unequal probabilities, non-coverage of the population, and non-response. • Data weights take the design weighting and incorporate characteristics of the population and the sample • Final Weights (_FINALWT) = Design Weight * some form of data weighting

  13. IN THE PAST…. • Post stratification was used as a weighting method. • It was based on known demographics of the population. • For BRFSS Post stratification included: Regions within states; Race/ Ethnicity (in detailed categories); Gender ; Age (in 7 categories) • Post-stratification forces the sum of the weighted frequencies to equal the population estimates for the region or state by race, age ,and gender. • Post stratification weights are applied to the responses, allowing for estimates of how groups of non-respondents would have answered survey questions.

  14. Beginning with the 2011 dataset, raking will succeed post stratification as the sole BRFSS statistical weighting method. • Allows for the incorporation of a now crucial variable – telephone source (landline or cell phone) into the BRFSS weighting methodology (called _LLCPWT) POST STRATIFICATION VS. RAKING

  15. Iterative Proportional Fitting (Raking) Weighting

  16. RAKING Rather than adjusting weights to categories, IPF adjusts for each dimension separately in an iterative process. The process will continue up to 75 times, or until data converges to Census estimates.

  17. Why Incorporate IPF Now? • Computer capacity has increased. • Cell phones are becoming larger percentage of the total number of calls. • Non-coverage with declining response rates makes weighting more important than ever.

  18. HOW DOES IT IMPACT US?

  19. HOW DOES IT IMPACT US?

  20. HOW DOES IT IMPACT US?

  21. HOW DOES IT IMPACT US?

  22. CONCLUSION • Raked weights impact differently for different variables. • Trend data and comparison cannot be established prior to 2009 due to changes in weighting procedures. • In 2008, there was only landline data available and hence raked weights reflected only landline. • With introduction of cell phones raking methods incorporated both landline as well as cell phone weights called _LLCPWT. • Starting 2011 CDC will weight data only using raking methods which implies establishing any trends is not feasible.

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