Docstoc

raj

Document Sample
raj Powered By Docstoc
					  People Saving People

                                      National Center for Statistics & Analysis




           Estimating Alcohol Involvement
                     in NHTSA’s
     Fatality Analysis Reporting System (FARS)


                                                    Rajesh (Raj) Subramanian
                                                Mathematical Analysis Division


29th Annual Traffic Records Forum, Denver, CO                                     1
                                                                     Definitions
  People Saving People

                                      National Center for Statistics & Analysis


    Blood Alcohol Concentration (BAC)
     - Measure of concentration of Alcohol (grams) in blood
        (deciliter).

    Alcohol Related Fatality
     - A fatality that occurs in a crash where at least one of the
        involved drivers,pedestrians or pedalcyclist has a BAC of 0.01
        or greater.
     - BAC is the only metric used to determine this.


    Intoxication
      - In most states, a person with a BAC of 0.08 or above.



29th Annual Traffic Records Forum, Denver, CO                                      2
                                                    Under-reporting of BAC values
  People Saving People
                                                  in FARS – A Historic Perspective
                                                             National Center for Statistics & Analysis


                              Proportion of Missing BAC Among
                     Actively Involved Persons – Drivers and Pedestrians
                     80
                     75
   Percent Missing




                     70
                     65
                     60
                     55
                     50
                     45
                     40
                     35
                          1982

                                 1983

                                        1984

                                               1985

                                                      1986

                                                             1987

                                                                    1988

                                                                            1989

                                                                                   1990

                                                                                          1991

                                                                                                 1992

                                                                                                        1993

                                                                                                               1994

                                                                                                                      1995

                                                                                                                             1996

                                                                                                                                    1997

                                                                                                                                           1998

                                                                                                                                                  1999

                                                                                                                                                         2000

                                                                                                                                                                2001
                                                                           Driver         Pedestrian

29th Annual Traffic Records Forum, Denver, CO                                                                                                                          3
                                                      Problems Posed by
  People Saving People
                                                            Missing BAC
                                      National Center for Statistics & Analysis


                              Hypothetical FARS Case
      Vehicle 1                                                             Vehicle 2
                                        Police has indicated
   Surviving Driver                                                      Fatally Injured
                                             that there is
       BAC=0                                                             Driver (BAC=?)
                                         probable cause to
   Fatally Injured                                                       Fatally Injured
                                       test driver of Vehicle 2
     Passenger                                                             Passenger
                                              for alcohol!



                                       Are the three fatalities
                                            in this crash
                                         Alcohol-Related?



               Cannot be Determined unless BAC is Estimated (Imputed)!

29th Annual Traffic Records Forum, Denver, CO                                              4
                                        Imputation of Missing BAC
  People Saving People

                                      National Center for Statistics & Analysis


        Estimate BAC only when it is missing
         - For any driver, pedestrian or pedalcyclist with missing
           BAC.
        Estimation is done based on crash, driver and vehicle
        related characteristics
         - Various predictors of alcohol are used.
         - Police-reported alcohol involvement (DRINKING) is
           highly predictive of presence/absence of alcohol
        Imputes actual value of BAC
         - Instead of a binary response (Yes/No).




29th Annual Traffic Records Forum, Denver, CO                                     5
                                          Candidates for Imputation
  People Saving People

                                      National Center for Statistics & Analysis



       Involved Driver with Missing BAC value.
         - Surviving or fatally injured Driver


       Involved Non-occupant with Missing BAC value
         - Surviving or fatally injured Pedestrian or
           Pedalcyclist




29th Annual Traffic Records Forum, Denver, CO                                     6
                              FARS Variables used to Impute
  People Saving People
                                               Missing BAC
                                      National Center for Statistics & Analysis



                                   Variables Used
 Police Reported Drinking License Status
 Age category             Previous Incidents (DWI, etc.)
 Gender                                         Day of the Week
 Use of restraint                               Time of the Day
 Injury severity                                Vehicle Role
                                                Relation to Roadway



29th Annual Traffic Records Forum, Denver, CO                                     7
                           Overview of Imputation Process
  People Saving People

                                      National Center for Statistics & Analysis

    Step 1
      - Choose set of variables that are significant in predicting
        dichotomous BAC (BAC=0 vs. BAC0)

    Step 2
      - Conditional on case having non-zero dichotomous BAC,
        choose set of variables that are significant in predicting
        continuous BAC.

    Step 3
      - Combine the results from Steps 1 and 2 into one general
        model and impute missing BAC.



29th Annual Traffic Records Forum, Denver, CO                                     8
                                                Imputation Domains and
  People Saving People
                                                          Process Flow
                                      National Center for Statistics & Analysis


    Steps 1 to 3 are performed within each vehicle class
      - Broad based vehicle categories like Cars, Utility Vehicles,
        Other LTVs, Minivans, Medium and Heavy Trucks, Motorcycles
        and all other vehicles.
      - Characteristics of Drivers for one class of vehicles may be
        radically different from those of another vehicle class (e.g.
        Minivans vs. Motorcycles)

    Non-occupants are treated as a separate class
      - Lesser number of predictor variables (No license status,
        restraint use, etc.).



29th Annual Traffic Records Forum, Denver, CO                                     9
                         Validation of Imputed BAC Values
  People Saving People

                                      National Center for Statistics & Analysis


    Impute BAC values intentionally set to Missing
     - Randomly set 25 percent of reported values to
       missing and impute these “missing” values
     - Compare imputed BAC values with the actual
       reported values
     - Repeat this test.


    Analysis on Imputation and probable cause for testing
     alcohol as indicated by Police Officer.
      - Suspected alcohol involvement, or the lack thereof,
        by the police officer.

29th Annual Traffic Records Forum, Denver, CO                                     10
                         Does it Work?… Validation Tests on
                         Estimating Alcohol-Related Fatalities
  People Saving People

                                      National Center for Statistics & Analysis


Rate of Driver and Non-occupant                      Randomly Set 25% of Known BAC
 Alcohol Testing for USA, 2001                         Values to missing and Impute


                          Known
                          = 44%
                                                                           Randomly Set 25%
                                                           75%
       Unknown                                                             to Missing
       = 56%




29th Annual Traffic Records Forum, Denver, CO                                             11
                                  25% Validation test on Alcohol-
                                     Related Fatalities, 2000
  People Saving People

                                      National Center for Statistics & Analysis


   Alcohol Related Fatalities as Percent of all fatalities
  in crashes involving persons with known BAC values
            Actual Values, 2000                                25% test, 2000


                         Alcohol Related                                 Alcohol Related
   No Alcohol                                           No Alcohol
   Related                     44%                      Related                   44%
          56%                    11,257                     56%                   11,245
                                                             14,473
             14,461



29th Annual Traffic Records Forum, Denver, CO                                              12
                          Analysis on Police-Reported Alcohol
  People Saving People
                                Involvement and Missing BAC
                                      National Center for Statistics & Analysis


    Alcohol Test Results changed from unknown value in
     the Annual Report File (ARF) to a reported value in the
     FINAL file.
      - Due to delay in test results being reported to FARS
      - Missing BAC values were imputed in the ARF.


    BAC values were reported for same cases in FINAL file.
      - Presents an opportunity to assess imputed BAC
        values.


29th Annual Traffic Records Forum, Denver, CO                                     13
                         Characteristics of Converted Cases –
  People Saving People
                         Police Reported Alcohol Involvement
                                      National Center for Statistics & Analysis




    Of all the converted cases
     - 35 percent indicated NO for police reported alcohol
        involvement (DRINKING)
     - 19 percent indicated YES for DRINKING.
     - 18 percent were coded as NOT REPORTED.
     - 29 percent were coded as UNKNOWN.




29th Annual Traffic Records Forum, Denver, CO                                     14
                         Comparison of Reported and Imputed
  People Saving People
                                   values for DRINKING=NO
                                      National Center for Statistics & Analysis




          -    95 percent of the FINAL reported BAC values were 0
          -    91 percent of the imputed BAC values in the ARF
               were 0.
          -    Reflecting an accuracy of close to 96 percent for this
               group.
          -    Small proportion of cases ended up with contrary
               result.



29th Annual Traffic Records Forum, Denver, CO                                     15
                         Comparison of Reported and Imputed
  People Saving People
                                  values for DRINKING=YES
                                      National Center for Statistics & Analysis




          -    88 percent of the FINAL reported BAC values were
               greater than zero.
          -    84 percent of the imputed BAC values in the ARF
               were greater than zero.
          -    Highlights importance of Police-Reported Alcohol
               Involvement in the imputation process.
          -    Imputed BAC values are highly correlated with
               Police-Reported Alcohol Involvement.


29th Annual Traffic Records Forum, Denver, CO                                     16
                         Comparison of Reported and Imputed values
                            for DRINKING=NOT Reported/UNKNOWN
  People Saving People

                                      National Center for Statistics & Analysis




          -    74 percent of FINAL reported BAC values were 0.
          -    70 percent of the imputed BAC values in the ARF
               were 0.
          -    Even when there is no mention of probable cause, or
               the lack thereof, for alcohol-testing, imputed BAC
               values are in close agreement with the reported
               BAC values.




29th Annual Traffic Records Forum, Denver, CO                                     17
                                                                    Conclusions
  People Saving People

                                      National Center for Statistics & Analysis


    States should not report alcohol involvement based only on
     reported values
      - Other cases may not have reported BAC values but may have
        characteristics that point to a high likelihood of alcohol
        involvement, or the lack thereof.
      - Varying rates of reporting year-to-year may not allow valid
        assessment of trends and progress.

    Imputation levels the field of comparison
      - Provides BAC values for all cases, so valid comparisons can
        be made.
      - Also provides statistical measures of uncertainty, etc.


29th Annual Traffic Records Forum, Denver, CO                                     18
                                                                    Conclusions
  People Saving People

                                      National Center for Statistics & Analysis


    Timeliness of Reporting to FARS
     - Annual Report File (ARF) is used for Annual
       Publications and Traffic Safety Facts and hence gets
       maximum attention.
     - Recommendation to FARS analysts to gather and
       code BAC values in time for the ARF.

    Imputation performs well
      - Even in cases where there is no mention of probable
        cause, or the lack thereof, for alcohol testing.

29th Annual Traffic Records Forum, Denver, CO                                     19
  People Saving People
                                    QUESTIONS?
                                      National Center for Statistics & Analysis




                                                                                   Jim
                                                                                  Hedlund



29th Annual Traffic Records Forum, Denver, CO                                          20

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:3
posted:12/18/2011
language:English
pages:20