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Mollie Pettit • @MollzMP
Racial Bias in Policing: an analysis of Illinois traffic stops data
Domino Data Science Pop-up • 11/14/2017
In 2016, over two million police traffic stops occurred in Illinois.
For each stop, an officer filled out this form.
They guessed the driver’s race…
recorded why thedriver was stopped…
whether they conducteda search…
if searched, whether or not contraband was found
and what action resulted.
other cities have looked at similar data…
Troopers Ticketing Fewer Overall, More Hispanics
The Disproportionate Risks of Driving While Black
Is there evidence of racial bias?
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
searched
not searched
warning
citationstopped
Let’s look at
Chicago(you are here)
UVA Racial Dot Map
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
searched
not searched
warning
citationstopped
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
searched
not searched
warning
citationstopped
31% black
29% hispanic
32% white
6% asian 2%
other
Chicago population~2.7 million total 1 square = ~310 people
61% black
20% hispanic
16% white
2% asian
1% other
31% black
29% hispanic
32% white
6% asian
2% other
2016 stop population~190,000 total
stopped113,287 stops 38,049 29,761 4,319
Chicago population
~2.7 million total1 square = ~310 people
1 square = ~20 people
Does this imply bias?
On its own, no.Why Not?
Why Not? 1) city population vs driving population
Why Not? 1) city population vs driving population
?
Why Not?2) officer-reported vs self-reported
I think this person is green
I self-report as blue
pink
blueorangegreen
traffic stops form
pinkblueorange
green
census form
Why Not?traffic stops form census
2) officer-reported vs self-reported
searched
not searched
warning
citationstopped
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
searched
not searched
warning
citationstopped
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
3,890 searches conducted / 187,133 stops
1,926 searches conducted / 113,287 stops
1,651 / 38,049
252 / 29,761
27 / 4,319
searched
1.7% of stopped black drivers searched
4.3%hispanic
0.8% white
0.6% asian
searched
1.7% 1,926 searches conducted / 113,287 stops
4.3% 1,651 / 38,049
0.8% 252 / 29,761
0.6% 27 / 4,319
white
black
hispanic
asian
white search rate
line of
equa
lity
equa
l rate
minority search
rate white drivers searched at higher rate
minority drivers searchedat higher rate
searched
white search rate
line of
equa
lity
equa
l rate
minority search
ratewhere’s Chicago?
searched
white search rate
line of
equa
lity
equa
l rate
minority search
rateblack
hispanic
asian
~ # of searches
searched
white search rate
min
ority
sea
rch
rate black hispanic asian
searched
How did Chicago compare to other police departments in Illinois?
white search rate
min
ority
sea
rch
rate
searched
Nearly all police departments searched black and hispanic drivers at higher rates than white and asian drivers
black hispanic asian
foundcontraband?
searched
not searched
warning
citationstopped
How often do police find contraband when a search is
conducted?
21.4% (54 / 252)
17.9% (296 / 1,651)
25.9% (7 / 27)
white
black
hispanic
asian
foundcontraband?
15.0% (289 search “hits”/1926 searches)
Chicago police had lower rates of contraband discovery for both black and hispanic drivers
foundcontraband?
~ # of search ‘hits’
white search ‘hit’ rate
minority search
‘hit’ rate
foundcontraband?
black hispanic asian
white search ‘hit’ ratemin
ority
sea
rch
‘hit’
rate
foundcontraband?
Most departments found contraband at lower rates when searching black and hispanic drivers
white search ‘hit’ ratemin
ority
sea
rch
‘hit’
rate black hispanic asian
searched
not searched
warning
citationstopped
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
searched
not searched
warning
citationstopped
Are minority drivers more likely to be stopped,
searched, or cited than white drivers?
citation
white
black
hispanic
asian
43.3% (12,896 / 29,761)
28.4% (32,171 citations / 113,287 stops)
43.7% (16,623 / 38,049)
40% (1,727 / 4,319)
Black drivers received more than half of all citations issued by Chicago police
citation white citation rate
minority citation
rate~ # of citations
citation
black hispanic asian
min
ority
cita
tion
rate
white citation rate
citation
black hispanic asian
min
ority
cita
tion
rate
white citation rate
Hispanic drivers were the most cited across Illinois, and Chicago is closer to equality than other
departments
Summary
- The proportion of black drivers in the stops population was twice the proportion of black residents in the city population
- There were more searches conducted of black drivers than any other group, but with the lowest contraband ‘hit’ rate
- Black drivers received a significantly lower ratio of citations than drivers of other races
- Hispanic drivers were significantly more likely to be searched in Chicago, and significantly more likely to be cited across Illinois
- Lots of factors to explore (we have, and will!)
in Chicagon in 2016:
Conclusion
Be thoughtful about the assumptions you’re making
Conclusion- Check out other work!
- Sharad Goel @ Stanford Open Policing Projectopenpolicing.stanford.edu
- Open Data Policing opendatapolicing.com
- Wanna check out the data or learn more? Contact us:
- [email protected] or @[email protected]
- Thanks to the ACLU of Illinois for providing us with the data
- Thanks to Datascope for help and feedback
Thank you for your kind attention!Conclusion- Check out other work!
- Sharad Goel @ Stanford Open Policing Projectopenpolicing.stanford.edu
- Open Data Policing opendatapolicing.com
- Wanna check out the data or learn more? Contact us:
- [email protected] or @[email protected]
- Thanks to the ACLU of Illinois for providing us with the data
- Thanks to Datascope for help and feedback
Just in case slides.
Splits by gender
white
black
hispanic
asian
search rate citation rate
Z-score for search rates
# of depts
What we expect for equal rate
Z-score for search rates
# of depts
z=10.7
z=27.3
z=-1.5
= Chicago
Z-score for search ‘hit’ rates
z=-2.6 z=-1.3 z=0.5
= Chicago
Over time…
Limitations
- Lack data about severity or incidents
- How fast was the speeder going?
- What kind of drug was found?
- Was the driver arrested?
- Was there any violence?
- Driving population vs census population
Steps forward / future work- Recommend additional data collection
- Granular location of each stop (GPS or address)
- Record type of drug, not just quantity
- Look into month by month time series
- Identify impact of police reforms (e.g. did new trainings affect number of stops?)
- Potential new reforms?
Chicago Stop Population 2015 vs 2016
61% black
20% hispanic
16% white
2% asian
1% other
49% black
24% hispanic
23% white
3% asian
2015 stop population
2016 stop population(~190,000 total)
(~86,000 total)
1% other
Is this significant?
p1 - rate for white personsp2 - rate for black personsn1 - total count for white personsn2 - total count for black persons
Scatter PlotsWhich metrics show bias?
What do all departments look like?How does an individual department compare?
Z-score HistogramsWhat does “significance” mean?Which departments are outliers?
Time Series PlotsHow have values change over time?
What do the raw values look like?