Dubuque PD Traffic Stop Analysis June 2017

Goal of the Study Look for Disproportionality in: (1) The decision to make a stop, and (2) The outcome of the stop

Definitions Disproportionality Stop Disproportionality Outcome A difference between police data and an expected value Stop Disproportionality A difference between racial demographics of police stops & a benchmark Outcome A difference in the racial demographics percentages of stop outcomes

Decision to stop We analyzed traffic stops made by the DPD between January 1st, 2015 and December 31st 2015. Goal: to compare the percentage of minority drivers stopped to a valid benchmark

Where the traffic stops were made

Traffic Observer Locations and Findings

Observation Locations and Numbers Correlation r for inter-surveyor observations within each location equals 0.63

Day verses Night Observations Night and day benchmark values are not significantly different from one another (mean difference = 0.001; t = 0.057; p = 0.95)

Correspondence between bench & census Census and Max benchmark values are not significantly different from one another (mean difference = 0.01; t = 0.47; p = 0.64)

Zone 1 adjustments Break into block groups

Final Benchmark Values in Observation Zones Census and Final benchmark values are not significantly different from one another (mean difference = 0.032; t = 0.995; p = 0.33)

DPD stops compared to the benchmark

Stops compared to benchmark

Weighted average across all zones for department as a whole A Low Level of Disproportionality Estimated margin of error is greater than +- 0.02.

Days vs Nights Days zone min % bench stops 1.1 0.12 0.11 170 1.2 0.08 219 1.3 0.17 0.15 393 4 0.02 0.05 214 5 0.24 0.2 330 6 101 7.1 0.23 64 7.2 0.1 106 9 0.10 176 11 0.03 349 99 0.07 1024 Nights zone min % bench stops 1.1 0.15 0.11 142 1.2 0.17 101 1.3 0.20 240 4 0.06 0.05 52 5 0.23 0.2 350 6 0.19 0.12 73 7.1 0.28 39 7.2 0.14 0.1 37 9 0.08 170 11 0.07 174 99 0.09 660

Disparity Index Summary Days Nights Dept. Disparity 0.009 0.045 0.024 Stops 3146 2038 5184

Discussion (1) The mean level of disproportionality for the department as a whole is low at roughly 2 percentage points above benchmark values. Within margin of error (2) The mean level of disproportionality for stops occurring during daylight hours is less than 1 percentage point above benchmark values. Within margin of error (3) The mean level of disproportionality for stops occurring during nighttime hours equals roughly 4.5 percentage points above benchmark values. Just outside margin of error

Stop Decision Conclusions Analyses found no significant levels of disproportionality for the department as whole. Specifically, Virtually no disproportionality in traffic stops during daylight hours (60% of all stops occur in daylight). Low levels of disproportionality during nighttime hours.

Citations, Arrests, and Search Requests Outcomes Citations, Arrests, and Search Requests

Citations Odds Ratio = 1.21 Minority drivers are slightly more likely to be ticketed on a traffic stop

Arrests (raw) Odds Ratio = 3.49 Minority drivers are more likely to be arrested on a traffic stop

Arrests (adjusted for warrants) Odds Ratio = 2.66 Minority drivers are more likely to be arrested on a traffic stop

Search Requests Odds Ratio = 6.08 Note: The number of search requests is very small. Therefore, the odds ratio may not be a valid indicator of disproportionality. The data show that the DPD request to search very infrequently. On average this occurs once every 260 stops.

Moving or Equipment Violation Reason for the Stop Moving or Equipment Violation

Moving Violations Odds Ratio = 0.69 (1.44) Whites are more likely to be stopped for moving violations

Equipment & Registration Violations Odds Ratio = 1.45 Minority drivers are more likely to be stopped for an equipment violation

Multivariate Analyses

The control variables used Time of day Gender Type of violation Age of driver Location of the stop

Logistic Regression Results The results of logistic regression show that controlling for important variables does not substantively change the results for citations, arrests and search requests. However, the results also show that racial differences in stops for moving & equipment violations become no longer significant

Conclusions Very low levels of disproportionality in traffic stops. Minority member drivers are not over represented in stops. No evidence of racial profiling Low levels of disproportionality in citations. White drivers and minority members drivers are nearly equally likely to receive a citation as the result of a stop Higher levels of disproportionality in arrests. The odds that minority member drivers are arrested on a traffic stop are about two-and-a-half times higher than for white drivers.