EDA Case Study: Baltimore Police Department Arrest data

EDA on Baltimore Police Dept Arrest Data-Fusion Analytics World


Data Scientists spend quite a lot of time cleaning and exploring data before creating predictive models, and it’s often the exploratory data analysis step that proves to be the most fruitful part of the data science pipeline.

In this article, we’ll take a walk through the process of understanding data through exploratory data analysis, where we’ll use Python as a tool to uncover interesting statistics of a real-world dataset.

Exploratory Data Analysis

Exploratory Data Analysis (EDA) is essentially the term used to describe the main characteristics of a dataset, often done very visually with charts and graphs.

The idea of using EDA to find insights into data was developed by John Tukey in 1977, and is now the subject of a copious amount of books and online courses.

A huge benefit of EDA over classical methods is that humans are great at recognizing patterns visually, and thus provides a shortcut to finding a good model worth trying.

Various graphical techniques are used in EDA, but are often quite simple:

  1. Simple data plots like scatter plots, bar diagrams, histograms, bihistograms, probability plots, block plots.
  2. Statistical plots like mean plots, standard deviation plots, box plots, and main effects plots of the raw data, which help in understanding the distribution of data.
  3. Various distribution-related plots that help us see how populations and samples differ and relate

To show you how to use some of these tools, I’m going to use Python to perform the analysis.

Required libraries

If you don’t have Python on your computer, you can use the Anaconda Python distribution to install most of the Python packages you need. Anaconda provides a simple double-click installer for your convenience.

This notebook uses several Python packages that come standard with the Anaconda Python distribution. The primary libraries that we’ll be using are:

  • pandas: Provides a DataFrame structure to store data in memory and work with it easily and efficiently.
  • matplotlib: Basic plotting library in Python; most other Python plotting libraries are built on top of it.

A Case Study

Data Source:

Let’s perform some EDA on real world data. The data we will consider is Baltimore Police Department Arrest data. The data is hosted on: Data set Source Baltimore Police Depratment’s website: Baltimore Police Department . Data consists of around 1,31,000 arrests made by the Baltimore Police Department.

This data represents the top arrest charge of those processed at Baltimore’s Central Booking & Intake Facility. This data does not contain those who have been processed through Juvenile Booking. The data set was originally created on October 18, 2011. The data set was last updated on November 18, 2016. It is updated on a monthly basis.

*Metadata: *

  • Arrest-ID
  • Age
  • Sex
  • Race
  • ArrestDate
  • ArrestTime
  • ArrestLocation
  • IncidentOffense
  • IncidentLocation
  • Charge
  • ChargeDescription
  • District
  • Post
  • Neighborhood
  • Location1(Location Coordinates)

* Let’s start by importing some libraries: *

import pandas as pd
import matplotlib.pyplot as plt
import numpy as np

%matplotlib inline
pd.options.display.max_rows = 100
pd.options.display.max_columns = 100

* First we will load the data and understand its dimension *

# reading the data from the csv file
data_BPD = pd.read_csv(‘./data/BPD_Arrests.csv’)

Dimension Load Data - EDA

**We are not loading the tables with the above dimensions as it will become huge, if you are interested, please connect with us on info@fusionanalyticsworld.com, we will share with you the tables separately

145654 rows × 15 columns

print(data_BPD.shape) # Get the shape of the data

(145654, 15)

So the data have * 145654 * observations and * 15 * variables.

Let’s have a sneak peek of data before we start our analysis:

data_BPD.describe(include=’all’) #Generates descriptive statistics

**Not loading the data here, you can contact separately

# Let’s see how many missing values we have in each variable
for column in data_BPD.columns:
print(column, data_BPD[column].isnull().sum())

Arrest 7687
Age 28
Sex 0
Race 0
ArrestDate 0
ArrestTime 0
ArrestLocation 59294
IncidentOffense 0
IncidentLocation 60594
Charge 17995
ChargeDescription 502
District 59323
Post 59342
Neighborhood 59324
Location 1 59250

We have NA values and missing labels in some variables. This is quite intuitive in real world data. We have to prepare our data taking care of all hindrances.

* Lets’ understand the Age distribution: *

plt.figure(figsize = (12,8)) # set the figure size
plt.xlabel(‘Age’, fontsize=18) # set the label of the x axis
plt.ylabel(‘Count’, fontsize=18) # set the label of the y axis
data_BPD[‘Age’].hist(bins = 50, color= ‘#2E2E2E’) # plotting

<matplotlib.axes._subplots.AxesSubplot at 0x7fc9318b88d0>Age Distribution Graph

As we see most of arrests are in the age group of * 20 -27. *

A cursory view of gender (Sex) distribution reveals the following:

plt.figure(figsize = (12,8))
plt.ylabel(‘Sex’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

data_BPD[‘Sex’].value_counts().plot.bar(stacked=True, color=’#2A0A29′)

<matplotlib.axes._subplots.AxesSubplot at 0x7fc931155518>Sex Distribution Graph

So the number of arrests is dominated by * male *.

* Let’s see how the race is distributed: *

plt.figure(figsize = (12,8))
plt.xlabel(‘Race’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

data_BPD[‘Race’].value_counts().plot.bar(stacked=True, color=’#071418′)

<matplotlib.axes._subplots.AxesSubplot at 0x7fc93103c208>Race Distribution Graph

So, we see from data people from B Race group are mostly arrested by police.

We have variables recording the * Arrest Date * and * Arrest Time *. We can use these variables to reveal some interesting patterns.

#First let’s convert our ArrestDate and ArrestTime columns to pandas.datetime type:
data_BPD[‘ArrestDate’] = pd.to_datetime(data_BPD[‘ArrestDate’], format=’%m/%d/%Y’)

# Some of the observations have hh.mm format so we adjust them to have the same format as the other observations hh:mm
data_BPD[‘ArrestTime’] = data_BPD[‘ArrestTime’].map(lambda s: s.replace(‘.’, ‘:’))
data_BPD[‘ArrestTime’] = pd.to_datetime(data_BPD[‘ArrestTime’], format=’%H:%M’)

plt.figure(figsize = (12,8))
.groupby(data_BPD.ArrestDate.dt.weekday_name) # first we group our data by the day of the week
.agg(‘count’) # We then compute the number of observation we have in each group
.plot.bar(stacked=True, color=’#0A2A0A’) # Plotting
plt.xlabel(‘Weekday of arrest’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92e6bcfd0>Weekly Arrest Data

We can see that * weekends * have fewer number of arrests.

plt.figure(figsize = (12,8))
.plot.bar(stacked=True, color=’#0A2229′)
plt.xlabel(‘Month of arrest’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc931d47470>Monthly Arrest Data

plt.figure(figsize = (12,8))
.plot.bar(stacked=True, color=’#220A29′)
plt.xlabel(‘Year of arrest’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc930b4b7b8>Yearly Arrest Data

Number of people arrested also show a * declining trend* year-on-year (2013-2017).

plt.figure(figsize = (12,8))
.plot.bar(stacked=True, color=’#610B0B’)
plt.xlabel(‘Hour of arrest’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc930afb748>Hourly Arrest Data

Now, we can see when most of the arrests occur in the * day*. There are fewer arrests in early morning.

Top 15 arrest locations are the following:

data_BPD[‘ArrestLocation’].value_counts()[:15].to_frame(‘Number of arrests’)

Number of arrests
200 N EUTAW ST 465
1500 RUSSELL ST 365
1600 W NORTH AVE 353
300 N EUTAW ST 251
600 LAURENS ST 230
1500 W NORTH AVE 210
1800 W PRATT ST 167

* Top 15 Incident Offense are the following:*

import re

# We remove symbols from IncidentOffense column and convert it to lowercase
data_BPD[‘IncidentOffense’] = data_BPD[‘IncidentOffense’].map(lambda s : re.sub(‘[^\w]’,”, s).lower())

data_BPD[‘IncidentOffense’].value_counts()[:15].to_frame(‘Number of incidents’)

Number of incidents
unknownoffense 96661
87narcotics 12391
4ecommonassault 7923
87onarcoticsoutside 2617
6clarcenyshoplifting 2444
79other 2046
4caggassltoth 1894
4baggassltcut 1633
24towedvehicle 1633
97searchseizure 1547
5aburgresforce 1045
55disorderlyperson 826
4daggasslthand 787
75destructofproperty 734
7astolenauto 705

* Top 15 Incident Locations are the following:*

data_BPD[‘IncidentLocation’].value_counts()[:15].to_frame(‘Number of incidents’)

Number of incidents
200 N Eutaw St 473
400 W Lexington St 240
300 N Eutaw St 235
1500 Russell St 222
400 E Baltimore St 221
1600 W North Av 200
400 W Saratoga St 197
600 Laurens St 175
600 Cherry Hill Rd 151
1200 W Pratt St 143
1800 W Pratt St 142
600 E Fayette St 135
3800 E Lombard St 129
200 N Conkling St 128
5100 Park Heights Av 127

* Top 15 charges: *

data_BPD[‘ChargeDescription’].value_counts()[:15].to_frame(‘Number of arrests’)

Number of arrests
Failure To Appear || Failure To Appear 15373
Unknown Charge 8508
Asslt-Sec Degree || Assault-Sec Degree 7769
Cds:Possess-Not Marihuana || Cds Violation 5873
Cds:Possess-Not Marihuana || Cds 3760
Violation Of Probation || Violation Of Probation 3041
Asslt-Sec Degree || Common Assault 2805
Cds: Poss Marihuana L/T 10 G || Cds Violation 1633
Asslt-Sec Degree || Assault 1486
Prostitution-General || Prostitution 1431
CDS 1422
Detain Only 1413
Asslt-First Degree || Assault-First Degree 1351

* Lets’ view the distribution of people arrested across different districts: *

plt.figure(figsize = (12,8))
data_BPD[‘District’].value_counts().plot.bar(stacked=True, color=’#2A0A29′)
plt.xlabel(‘District’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘District Distribution’, fontsize=20)

<matplotlib.text.Text at 0x7fc930071908>

District Distribution Graph

Comparatively, * Southern district * has more number of arrests.

Distribution of Arrest Year & Arrest month

data_BPD[‘year’] = data_BPD[‘ArrestDate’].dt.year
data_BPD[‘month’] = data_BPD[‘ArrestDate’].dt.month
(data_BPD.pivot_table(‘Arrest’, index=’year’, columns=’month’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))

plt.xlabel(‘Year’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Distribution of Arrest Year and Arrest month’, fontsize = 22)

<matplotlib.text.Text at 0x7fc92ff46278>Arrest Year and Month Graph

hour_period_map = {
0 : ‘0-3’,
1 : ‘0-3’,
2 : ‘0-3’,
3 : ‘0-3’,
4 : ‘4-7’,
5 : ‘4-7’,
6 : ‘4-7’,
7 : ‘4-7’,
8 : ‘8-11’,
9 : ‘8-11’,
10 : ‘8-11’,
11 : ‘8-11’,
12 : ’12-15′,
13 : ’12-15′,
14 : ’12-15′,
15 : ’12-15′,
16 : ’16-19′,
17 : ’16-19′,
18 : ’16-19′,
19 : ’16-19′,
20 : ’20-23′,
21 : ’20-23′,
22 : ’20-23′,
23 : ’20-23′,
data_BPD[‘period_in_day’] = data_BPD[‘ArrestTime’].map(lambda time : hour_period_map[time.hour])
data_BPD[‘weekday’] = data_BPD[‘ArrestDate’].dt.weekday_name
(data_BPD.pivot_table(‘Arrest’, index=’weekday’, columns=’period_in_day’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))

plt.xlabel(‘Day in the week’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Distribution of arrests Weekdays and Time’, fontsize = 22)

<matplotlib.text.Text at 0x7fc92e380d68>Arrest Weekdays and Time Graph

def age_cat_map(age):
if age < 20 :
return ‘0-20’
elif age < 40 :
return ’20-40′
elif age < 60 :
return ’40-60′
elif age < 60 :
return ’60-80′
else :
return ’80-‘

data_BPD[‘age_cat’] = data_BPD[‘Age’].map(age_cat_map)
(data_BPD.pivot_table(‘Arrest’, index=’Sex’, columns=’age_cat’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))
plt.title(‘Distribution of Arrests Age & Sex’, fontsize = 22)
plt.xlabel(‘Sex’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92e24e6a0>

Arrest Age and Sex Graph

(data_BPD.pivot_table(‘Arrest’, index=’Race’, columns=’age_cat’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))

plt.xlabel(‘Race’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Distribution of Arrests Age & Race’, fontsize = 22)

<matplotlib.text.Text at 0x7fc92e192a58>

Arrest Age and Race Graph

(data_BPD.pivot_table(‘Arrest’, index=’year’, columns=’Sex’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))
plt.title(‘Distribution of Arrests Year & Sex’, fontsize = 22)
plt.xlabel(‘Year’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92e069668>

Arrest Year and Sex Graph

(data_BPD.pivot_table(‘Arrest’, index=’year’, columns=’Race’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))
plt.title(‘Distribution of Arrests Year & Race’, fontsize = 22)

plt.xlabel(‘Year’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92dfb8748>Arrest Year and Race Graph

(data_BPD.pivot_table(‘Arrest’, index=’year’, columns=’District’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’,stacked=True, figsize=(18,10))
plt.title(‘Distribution of Arrests Year & District’, fontsize = 22)

plt.xlabel(‘Year’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92de24278>

Arrest Year and District Graph

(data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’Race’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’, stacked = True, figsize=(18,10))
plt.title(‘Distribution of arrests age and race in a histogram’, fontsize = 22)

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92dcb7e10>Arrest Age and Race Histogram

df = data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’Race’, aggfunc = ‘count’, fill_value=0)

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)

<matplotlib.text.Text at 0x7fc92d9d8dd8>Count vs Age Plot

(data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’District’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’, stacked = True, figsize=(18,10))

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Arrests across district and age’, fontsize = 22)

<matplotlib.text.Text at 0x7fc92d346320>Arrest Across District and Age Histogram

(data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’month’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’, stacked = True, figsize=(18,10))

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Arrests Age Month’, fontsize = 22)

<matplotlib.text.Text at 0x7fc924ede518>Arrest Age Month Histogram

(data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’year’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’, stacked = True, figsize=(18,10))

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Arrests Age Year’, fontsize = 22)

<matplotlib.text.Text at 0x7fc9242057b8>Arrest Age Year Histogram

(data_BPD.pivot_table(‘Arrest’, index=’Age’, columns=’weekday’, aggfunc = ‘count’, fill_value=0)
.plot(kind=’bar’, stacked = True, figsize=(18,10))

plt.xlabel(‘Age’, fontsize=18)
plt.ylabel(‘count’, fontsize=18)
plt.title(‘Distribution of Arrests on age and Weekdays’, fontsize = 22)

<matplotlib.text.Text at 0x7fc91efe0908>Arrest Age and Weekdays Distribution

We have seen the distributions of number of arrests across various variables and the interactions among them.

End Notes:

We can appreciate the role of exploratory data analysis in understanding the patterns of data and preparation of hypothesis. These serve as building blocks for further analysis. We have seen various plots to understand the data. For better understanding we have analyzed a real world data set. Stakeholders may take few of the decisions on prevention of crime and deployment of personnel after having a glance on the plots. A resourceful analysis can help taking complex decisions. We can use Predictive analytics and various optimization techniques for taking such decisions.

Author: Mohamed R., a Machine Learning and Signal Processing expert and part-time author at LearnDataSci

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Kalyan Banga224 Posts

I am Kalyan Banga, a Post Graduate in Business Analytics from Indian Institute of Management (IIM) Calcutta, a premier management institute, ranked best B-School in Asia in FT Masters management global rankings. I have spent 14 years in field of Research & Analytics.

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