National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 1 Estimating Alcohol Involvement in NHTSA’s Fatality Analysis Reporting System (FARS) Rajesh (Raj) Subramanian Mathematical Analysis Division

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 2 Definitions  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 3 Under-reporting of BAC values in FARS – A Historic Perspective Proportion of Missing BAC Among Actively Involved Persons – Drivers and Pedestrians

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 4 Problems Posed by Missing BAC Are the three fatalities in this crash Alcohol-Related? Vehicle 1 Surviving Driver BAC=0 Fatally Injured Passenger Vehicle 2 Fatally Injured Driver (BAC=?) Fatally Injured Passenger Hypothetical FARS Case Police has indicated that there is probable cause to test driver of Vehicle 2 for alcohol! Cannot be Determined unless BAC is Estimated (Imputed)!

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 5 Imputation of Missing BAC  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).

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 6 Candidates for Imputation  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

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 7 FARS Variables used to Impute Missing BAC Variables Used Police Reported DrinkingLicense Status Age categoryPrevious Incidents (DWI, etc.) GenderDay of the Week Use of restraintTime of the Day Injury severityVehicle Role Relation to Roadway

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 8 Overview of Imputation Process  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 9 Imputation Domains and Process Flow  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.).

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 10 Validation of Imputed BAC Values  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 11 Does it Work?… Validation Tests on Estimating Alcohol-Related Fatalities Known = 44% Unknown = 56% Rate of Driver and Non-occupant Alcohol Testing for USA, 2001 Randomly Set 25% to Missing 75% Randomly Set 25% of Known BAC Values to missing and Impute

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 12 25% Validation test on Alcohol- Related Fatalities, 2000 Alcohol Related Fatalities as Percent of all fatalities in crashes involving persons with known BAC values Actual Values, % test, 2000 Alcohol Related No Alcohol Related Alcohol Related No Alcohol Related 14,461 11,257 14,473 11,245

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 13 Analysis on Police-Reported Alcohol Involvement and Missing BAC  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 14 Characteristics of Converted Cases – Police Reported Alcohol Involvement  Of all the converted cases - 35 percent indicated NO for police reported alcohol involvement (DRINKING) - 19 percent indicated YES for DRINKING percent were coded as NOT REPORTED percent were coded as UNKNOWN.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 15 Comparison of Reported and Imputed values for DRINKING=NO - 95 percent of the FINAL reported BAC values were 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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 16 Comparison of Reported and Imputed values for DRINKING=YES - 88 percent of the FINAL reported BAC values were greater than zero 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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 17 Comparison of Reported and Imputed values for DRINKING=NOT Reported/UNKNOWN - 74 percent of FINAL reported BAC values were 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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 18 Conclusions  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 19 Conclusions  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.

National Center for Statistics & Analysis People Saving People 29 th Annual Traffic Records Forum, Denver, CO 20 QUESTIONS? Jim Hedlund