AS1-1 An Analytical Detective

---

Section 1 - Loading the Data

1.1

How many rows of data (observations) are in this dataset?

nrow(D)

[1] 191641

1.2

How many variables are in this dataset?

ncol(D)

[1] 11

1.3

Using the “max” function, what is the maximum value of the variable “ID”?

max(D$ID)

[1] 9181151

1.4

What is the minimum value of the variable “Beat”?

min(D$Beat)

[1] 111

1.5

How many observations have value TRUE in the Arrest variable (this is the number of crimes for which an arrest was made)?

summary(D$Arrest) #15536

   Mode   FALSE    TRUE 
logical  176105   15536 

1.6

How many observations have a LocationDescription value of ALLEY?

summary(D$LocationDescription == "ALLEY") #2308

   Mode   FALSE    TRUE 
logical  189333    2308 

Section 2 - Understanding Dates in R

In many datasets, like this one, you have a date field. Unfortunately, R does not automatically recognize entries that look like dates. We need to use a function in R to extract the date and time. Take a look at the first entry of Date (remember to use square brackets when looking at a certain entry of a variable).

2.1

In what format are the entries in the variable Date?

• Month/Day/Year Hour:Minute
• Day/Month/Year Hour:Minute
• Hour:Minute Month/Day/Year
• Hour:Minute Day/Month/Year

head(D$Date) #Month/Day/Year Hour:Minute

[1] 12/31/12 23:15 12/31/12 22:00 12/31/12 22:00 12/31/12 22:00
[5] 12/31/12 21:30 12/31/12 20:30
131680 Levels: 10/10/01 0:00 10/10/01 0:01 10/10/01 0:30 ... 9/9/12 9:50

2.2

Now, let’s convert these characters into a Date object in R. In your R console, type

DateConvert = as.Date(strptime(mvt$Date, "%m/%d/%y %H:%M"))

This converts the variable “Date” into a Date object in R. Take a look at the variable DateConvert using the summary function.

What is the month and year of the median date in our dataset? Enter your answer as “Month Year”, without the quotes. (Ex: if the answer was 2008-03-28, you would give the answer “March 2008”, without the quotes.)

DateConvert = as.Date(strptime(D$Date, "%m/%d/%y %H:%M"))
median(DateConvert)

[1] "2006-05-21"

# May 2006

2.3

Now, let’s extract the month and the day of the week, and add these variables to our data frame mvt. We can do this with two simple functions. Type the following commands in R:

mvt$Month = months(DateConvert)

mvt$Weekday = weekdays(DateConvert)

This creates two new variables in our data frame, Month and Weekday, and sets them equal to the month and weekday values that we can extract from the Date object. Lastly, replace the old Date variable with DateConvert by typing:

mvt$Date = DateConvert

Using the table command, answer the following questions.

In which month did the fewest motor vehicle thefts occur?

D$Month = months(DateConvert)
D$Weekday = weekdays(DateConvert)
D$Date = DateConvert
table(D$Month) #February 13511

    April    August  December  February   January      July      June 
    15280     16572     16426     13511     16047     16801     16002 
    March       May  November   October September 
    15758     16035     16063     17086     16060 

2.4

On which weekday did the most motor vehicle thefts occur?

table(D$Weekday) #Sunday 26316

   Friday    Monday  Saturday    Sunday  Thursday   Tuesday Wednesday 
    29284     27397     27118     26316     27319     26791     27416 

2.5

Each observation in the dataset represents a motor vehicle theft, and the Arrest variable indicates whether an arrest was later made for this theft. Which month has the largest number of motor vehicle thefts for which an arrest was made?

table(D$Month,D$Arrest) #January 1435

           
            FALSE  TRUE
  April     14028  1252
  August    15243  1329
  December  15029  1397
  February  12273  1238
  January   14612  1435
  July      15477  1324
  June      14772  1230
  March     14460  1298
  May       14848  1187
  November  14807  1256
  October   15744  1342
  September 14812  1248

Section 3 - Visualizing Crime Trends

3.1

Now, let’s make some plots to help us better understand how crime has changed over time in Chicago. Throughout this problem, and in general, you can save your plot to a file. For more information, this website very clearly explains the process.

First, let’s make a histogram of the variable Date. We’ll add an extra argument, to specify the number of bars we want in our histogram. In your R console, type

hist(mvt$Date, breaks=100)

hist(D$Date, breaks=100)

Looking at the histogram, answer the following questions.

In general, does it look like crime increases or decreases from 2002 - 2012?

• Increases
• Decreases

#Decreases

In general, does it look like crime increases or decreases from 2005 - 2008?

• Increases
• Decreases

#decreases

3.2

Now, let’s see how arrests have changed over time. Create a boxplot of the variable “Date”, sorted by the variable “Arrest” (if you are not familiar with boxplots and would like to learn more, check out this tutorial). In a boxplot, the bold horizontal line is the median value of the data, the box shows the range of values between the first quartile and third quartile, and the whiskers (the dotted lines extending outside the box) show the minimum and maximum values, excluding any outliers (which are plotted as circles). Outliers are defined by first computing the difference between the first and third quartile values, or the height of the box. This number is called the Inter-Quartile Range (IQR). Any point that is greater than the third quartile plus the IQR or less than the first quartile minus the IQR is considered an outlier.

Does it look like there were more crimes for which arrests were made in the first half of the time period or the second half of the time period? (Note that the time period is from 2001 to 2012, so the middle of the time period is the beginning of 2007.)

• First half
• Second half

3.3

Let’s investigate this further. Use the table function for the next few questions.

For what proportion of motor vehicle thefts in 2001 was an arrest made?

Note: in this question and many others in the course, we are asking for an answer as a proportion. Therefore, your answer should take a value between 0 and 1.

table(D$Year)

 2001  2002  2003  2004  2005  2006  2007  2008  2009  2010  2011  2012 
20669 18753 16657 16862 16484 16098 14280 14445 12167 15497 15637 14092 

ans = 20669/nrow(D)
ans

[1] 0.1078527

3.4

For what proportion of motor vehicle thefts in 2007 was an arrest made?

ans = 14280/nrow(D)
ans

[1] 0.07451433

3.5

For what proportion of motor vehicle thefts in 2012 was an arrest made?

ans = 14092/nrow(D)
ans

[1] 0.07353333

Since there may still be open investigations for recent crimes, this could explain the trend we are seeing in the data. There could also be other factors at play, and this trend should be investigated further. However, since we don’t know when the arrests were actually made, our detective work in this area has reached a dead end.

Section 4 - Popular Locations

4.1

Analyzing this data could be useful to the Chicago Police Department when deciding where to allocate resources. If they want to increase the number of arrests that are made for motor vehicle thefts, where should they focus their efforts?

We want to find the top five locations where motor vehicle thefts occur. If you create a table of the LocationDescription variable, it is unfortunately very hard to read since there are 78 different locations in the data set. By using the sort function, we can view this same table, but sorted by the number of observations in each category. In your R console, type:

sort(table(mvt$LocationDescription))

Which locations are the top five locations for motor vehicle thefts, excluding the “Other” category? You should select 5 of the following options.

• Bank
• Gas Station
• Hotel/Motel
• Street
• Car Wash
• Restaurant
• Parking Lot/Garage (Non-Residential)
• Alley
• Driveway (Residential)
• Vacant Lot/Land

head(sort(table(D$LocationDescription),decreasing = TRUE))

                        STREET PARKING LOT/GARAGE(NON.RESID.) 
                        156564                          14852 
                         OTHER                          ALLEY 
                          4573                           2308 
                   GAS STATION         DRIVEWAY - RESIDENTIAL 
                          2111                           1675 

4.2

Create a subset of your data, only taking observations for which the theft happened in one of these five locations, and call this new data set “Top5”. To do this, you can use the | symbol. In lecture, we used the & symbol to use two criteria to make a subset of the data. To only take observations that have a certain value in one variable or the other, the | character can be used in place of the & symbol. This is also called a logical “or” operation.

Alternately, you could create five different subsets, and then merge them together into one data frame using rbind.

How many observations are in Top5?

Top5 = subset(D,LocationDescription == "STREET" | 
                LocationDescription =="PARKING LOT/GARAGE(NON.RESID.)"|
              LocationDescription =="OTHER","ALLEY"|
              LocationDescription =="GAS STATION"|
              LocationDescription =="DRIVEWAY - RESIDENTIAL")

Error in "ALLEY" | LocationDescription == "GAS STATION" : 
  operations are possible only for numeric, logical or complex types

4.3

R will remember the other categories of the LocationDescription variable from the original dataset, so running table(Top5$LocationDescription) will have a lot of unnecessary output. To make our tables a bit nicer to read, we can refresh this factor variable. In your R console, type:

Top5$LocationDescription = factor(Top5$LocationDescription)

If you run the str or table function on Top5 now, you should see that LocationDescription now only has 5 values, as we expect.

Use the Top5 data frame to answer the remaining questions.

One of the locations has a much higher arrest rate than the other locations. Which is it? Please enter the text in exactly the same way as how it looks in the answer options for Problem 4.1.

table(Top5$LocationDescription)

                         ALLEY                    GAS STATION 
                          2308                           2111 
                         OTHER PARKING LOT/GARAGE(NON.RESID.) 
                          4573                          14852 
                        STREET 
                        156564 

4.4

On which day of the week do the most motor vehicle thefts at gas stations happen? (Monday~Sunday)

table(Top5$LocationDescription,Top5$Weekday) #ans: sunday

                                
                                 Friday Monday Saturday Sunday Thursday
  ALLEY                             385    320      341    307      315
  GAS STATION                       332    280      338    336      282
  OTHER                             751    704      597    516      687
  PARKING LOT/GARAGE(NON.RESID.)   2331   2128     2199   1936     2082
  STREET                          23773  22305    22175  21756    22296
                                
                                 Tuesday Wednesday
  ALLEY                              323       317
  GAS STATION                        270       273
  OTHER                              637       681
  PARKING LOT/GARAGE(NON.RESID.)    2073      2103
  STREET                           21888     22371

4.5

On which day of the week do the fewest motor vehicle thefts in residential driveways happen?(Monday~Sunday)

table(D$LocationDescription == "DRIVEWAY - RESIDENTIAL",D$Weekday) #ans:Saturday

       
        Friday Monday Saturday Sunday Thursday Tuesday Wednesday
  FALSE  29027  27142    26916  26095    27056   26548     27182
  TRUE     257    255      202    221      263     243       234