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:

- molliempettit@gmail.com or @MollzMPchris.kucharczyk@gmail.com

- 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:

- molliempettit@gmail.com or @MollzMPchris.kucharczyk@gmail.com

- 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?