1. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL Session 38 Alcohol Imputation Model Why Impute? Joseph (Joe) M. Tessmer Mathematical Analysis Division

2. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 2 Why?

3. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 3 There is a problem in Alcohol Reporting

4. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 4 Wide range of BAC reporting of drivers and non-occupants by states  Levels of reporting alcohol test results for drivers and non-occupants involved in fatal crashes ranged by states from: Less than 12 Percent to More than 86 Percent

5. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 5 Why Impute ?  Reduce Potential Biases in Estimates  14 % to 88 % of the BAC test results are missing in FARS – dependent on the state  Nationally, approximately 60 % of BAC data are missing for drivers and non-occupants

6. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 6 Why Impute ?  If the individuals selected for BAC level testing is not a random sample, the estimates will be biased  Often only drivers suspected of a high BAC are tested  We would over-estimate BAC levels  44 % of tested individuals had BAC > 0

7. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 7 Current Imputation Procedure  3 Level discriminant analysis  For each missing BAC number calculate the probability that  1) BAC = 0  2) 0 < BAC < 0.1  3) 0.1 <= BAC  Note the probabilities add to 1

8. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 8 Current Imputation Procedure  Provides some useful information  It was a major step forward when introduced in 1986  It is a rigid procedure and can not be used to quantify the effect of the current 0.08 BAC legislation

9. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 9 Current Imputation Procedure  Results can not be used as input to other types of analysis  Can not be used as an independent variable in crash analysis  Can be used as a weight

10. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 10 Why Change to Multiple Imputation ?  State of the art solution  Imputed values are actual BAC levels which can be used in additional analysis.

11. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 11 Why Change to Multiple Imputation ?  Improve fidelity of results  Permits analysis at any level of BAC  Old technique uses the probability that value falls within one of three ranges [More difficult to use.]

12. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 12 Why Change to Multiple Imputation ?  Can calculate the standard error of the estimates.  Achieve greater confidence in results (narrower confidence limits)

13. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 13 Multiple Imputation Advantages  Uses several characteristics of the crash and of the driver or non-occupant to estimate BAC levels.

14. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 14 Example 1  Driver Characteristics  Female Driver  36 years old  Seat belt used  Crash Characteristics  8:20 a.m. Tuesday in October  3 passengers all children in vehicle  2 Vehicles involved  Police reported no drinking and no BAC data  Estimated BAC = 0.0

15. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 15 Example 2  Driver Characteristics  Male Driver  23 years old  Seatbelt not used  Crash Characteristics  2:10 a.m. Saturday in July  No passengers  Single vehicle crash  Police reported drinking but no BAC data Estimated BAC = 0.14

16. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 16 Example 3  Driver Characteristics  Male Driver  23 years old  Seatbelt not used  Crash Characteristics  2:10 a.m. Saturday in July  No passengers  Single vehicle crash  Police reported no drinking and no BAC data  Estimated BAC = 0.00

17. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 17  Multiple Imputation is a lot of work...  Uses several characteristics of the crash and of the driver or non-occupant to estimate 10 BAC levels for each case w/missing BAC.  Does it work?  Are the numbers right?

18. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 18  Yes!!

19. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 19 Verification Test  Select a year of FARS data  Restrict data to known BAC data  Randomly recoded 25 % of known BAC “data missing”

20. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 20 Verification Test  Impute the “missing data”, based on 75% remaining data  Compare estimates vs. the actual data  Repeat test.

21. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 21 Verification Test BAC Estimates for the 25% Removed 19931995 0.01+0.1+0.01+0.1+ Percent Total Drivers Involved Known46.036.943.734.7 MI45.737.743.635.7 Percent Total Drivers Killed (Daytime) Known20.615.921.015.5 MI21.518.021.517.7

22. National Center for Statistics & Analysis People Saving People 28 th Annual Traffic Records Forum, Orlando, FL 22 QUESTIONS?