Homework Assignment: Analyzing NYC Flight Data

This homework assignment uses the flights dataset from the nycflights13 package, which contains real-world data on over 336,000 flights departing from New York City airports (JFK, LGA, EWR) in 2013. The dataset includes variables such as departure and arrival times (with date components), airline carrier (categorical), origin and destination airports (categorical), delays (with missing values for cancelled flights), distance, and more. It is sourced from the US Bureau of Transportation Statistics.

Objectives

This assignment reinforces the Week 4 topics:

  • Parsing and manipulating dates/times using lubridate.
  • Creating and analyzing time series with zoo.
  • Working with factors, inspecting levels, and recoding them.
  • Identifying and handling missing data (e.g., removal, imputation).

All questions (except the final reflection) require you to write and run R code to solve them. Submit your URL for your RPubs. Make sure to comment your code, along with key outputs (e.g., summaries, plots, or tables). Use the provided setup code to load the data.

Setup

Install and load the necessary packages if not already done:

#install.packages(c("nycflights13", "dplyr", "lubridate", "zoo", "forcats"))  # If needed
library(nycflights13)
## Warning: package 'nycflights13' was built under R version 4.4.3
library(dplyr)
## Warning: package 'dplyr' was built under R version 4.4.3
## 
## Attaching package: 'dplyr'
## The following objects are masked from 'package:stats':
## 
##     filter, lag
## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union
library(lubridate)
## Warning: package 'lubridate' was built under R version 4.4.3
## 
## Attaching package: 'lubridate'
## The following objects are masked from 'package:base':
## 
##     date, intersect, setdiff, union
library(zoo)
## Warning: package 'zoo' was built under R version 4.4.3
## 
## Attaching package: 'zoo'
## The following objects are masked from 'package:base':
## 
##     as.Date, as.Date.numeric
library(forcats)  # For factor recoding; base R alternatives are acceptable
## Warning: package 'forcats' was built under R version 4.4.3
data(flights)  # Load the dataset

Explore the data briefly with str(flights) and head(flights) to understand the structure. Note: Dates are in separate year, month, day columns; times are in dep_time and arr_time (as integers like 517 for 5:17 AM).

#Explore your data here
#inspect structure and first rows to understand the data
str(flights) 
## tibble [336,776 × 19] (S3: tbl_df/tbl/data.frame)
##  $ year          : int [1:336776] 2013 2013 2013 2013 2013 2013 2013 2013 2013 2013 ...
##  $ month         : int [1:336776] 1 1 1 1 1 1 1 1 1 1 ...
##  $ day           : int [1:336776] 1 1 1 1 1 1 1 1 1 1 ...
##  $ dep_time      : int [1:336776] 517 533 542 544 554 554 555 557 557 558 ...
##  $ sched_dep_time: int [1:336776] 515 529 540 545 600 558 600 600 600 600 ...
##  $ dep_delay     : num [1:336776] 2 4 2 -1 -6 -4 -5 -3 -3 -2 ...
##  $ arr_time      : int [1:336776] 830 850 923 1004 812 740 913 709 838 753 ...
##  $ sched_arr_time: int [1:336776] 819 830 850 1022 837 728 854 723 846 745 ...
##  $ arr_delay     : num [1:336776] 11 20 33 -18 -25 12 19 -14 -8 8 ...
##  $ carrier       : chr [1:336776] "UA" "UA" "AA" "B6" ...
##  $ flight        : int [1:336776] 1545 1714 1141 725 461 1696 507 5708 79 301 ...
##  $ tailnum       : chr [1:336776] "N14228" "N24211" "N619AA" "N804JB" ...
##  $ origin        : chr [1:336776] "EWR" "LGA" "JFK" "JFK" ...
##  $ dest          : chr [1:336776] "IAH" "IAH" "MIA" "BQN" ...
##  $ air_time      : num [1:336776] 227 227 160 183 116 150 158 53 140 138 ...
##  $ distance      : num [1:336776] 1400 1416 1089 1576 762 ...
##  $ hour          : num [1:336776] 5 5 5 5 6 5 6 6 6 6 ...
##  $ minute        : num [1:336776] 15 29 40 45 0 58 0 0 0 0 ...
##  $ time_hour     : POSIXct[1:336776], format: "2013-01-01 05:00:00" "2013-01-01 05:00:00" ...
head(flights)
## # A tibble: 6 × 19
##    year month   day dep_time sched_dep_time dep_delay arr_time sched_arr_time
##   <int> <int> <int>    <int>          <int>     <dbl>    <int>          <int>
## 1  2013     1     1      517            515         2      830            819
## 2  2013     1     1      533            529         4      850            830
## 3  2013     1     1      542            540         2      923            850
## 4  2013     1     1      544            545        -1     1004           1022
## 5  2013     1     1      554            600        -6      812            837
## 6  2013     1     1      554            558        -4      740            728
## # ℹ 11 more variables: arr_delay <dbl>, carrier <chr>, flight <int>,
## #   tailnum <chr>, origin <chr>, dest <chr>, air_time <dbl>, distance <dbl>,
## #   hour <dbl>, minute <dbl>, time_hour <dttm>
#Show column names for quick reference 
colnames(flights) 
##  [1] "year"           "month"          "day"            "dep_time"      
##  [5] "sched_dep_time" "dep_delay"      "arr_time"       "sched_arr_time"
##  [9] "arr_delay"      "carrier"        "flight"         "tailnum"       
## [13] "origin"         "dest"           "air_time"       "distance"      
## [17] "hour"           "minute"         "time_hour"

Question 1: Creating Dates with lubridate

Create a column dep_datetime by combining year, month, day, and dep_time into a POSIXct datetime using lubridate. (Hint: Use make_datetime function to combine: year, month, day, for hour and min use division, e.g., hour = dep_time %/% 100, min = dep_time %% 100.)

Show the first 5 rows of flights with dep_datetime.

Output: First 5 rows showing year, month, day, dep_time, and dep_datetime.

# Remove rows with missing dep_time for this step to avoid parsing problems

flights_q1 <- flights %>%
filter(!is.na(dep_time)) %>%
mutate(
dep_datetime = make_datetime(
year = year,
month = month,
day = day,
hour = dep_time %/% 100,  # e.g., 517 -> 5
min  = dep_time %% 100    # e.g., 517 -> 17
)
)

# Output: first 5 rows with the new column

head(flights_q1 %>% select(year, month, day, dep_time, dep_datetime), 5)
## # A tibble: 5 × 5
##    year month   day dep_time dep_datetime       
##   <int> <int> <int>    <int> <dttm>             
## 1  2013     1     1      517 2013-01-01 05:17:00
## 2  2013     1     1      533 2013-01-01 05:33:00
## 3  2013     1     1      542 2013-01-01 05:42:00
## 4  2013     1     1      544 2013-01-01 05:44:00
## 5  2013     1     1      554 2013-01-01 05:54:00

Question 2: Simple Date Manipulations with lubridate

Using dep_datetime from Question 1, create a column weekday with the day of the week (e.g., “Mon”) using wday(dep_datetime, label = TRUE). Use table() to show how many flights occur on each weekday.

Output: The table of flight counts by weekday.

# Add weekday (Mon, Tue, ...) using dep_datetime from Q1

flights_q2 <- flights_q1 %>%
mutate(weekday = wday(dep_datetime, label = TRUE))

# Output: table of counts by weekday

table(flights_q2$weekday)
## 
##   Sun   Mon   Tue   Wed   Thu   Fri   Sat 
## 45643 49468 49273 48858 48654 48703 37922

Question 3: Time Series with zoo

Filter for flights from JFK (origin == “JFK”) and create a zoo time series of departure delays (dep_delay) by dep_datetime. Plot the time series (use plot()). (Hint: Use a subset to avoid memory issues, e.g., first 1000 JFK flights using `slice_head().)

Output: The time series plot.

# Subset: JFK flights ordered by datetime, first 1000 rows

jfk_subset <- flights_q1 %>%
filter(origin == "JFK") %>%
arrange(dep_datetime) %>%
slice_head(n = 1000)

# Create a zoo time series

dep_delay_series <- zoo(jfk_subset$dep_delay, order.by = jfk_subset$dep_datetime)
## Warning in zoo(jfk_subset$dep_delay, order.by = jfk_subset$dep_datetime): some
## methods for "zoo" objects do not work if the index entries in 'order.by' are
## not unique
# Plot the time series

plot(dep_delay_series,
main = "JFK Departure Delays (first 1000 flights)",
xlab = "Departure datetime",
ylab = "Departure delay (minutes)")

Question 4: Working with Factors

Convert the origin column (airports: “JFK”, “LGA”, “EWR”) to a factor called origin_factor. Show the factor levels with levels() and create a frequency table with table(). Make a bar plot of flights by airport using barplot().

Output: The levels, frequency table, and bar plot.

# Convert origin to factor

flights$origin_factor <- as.factor(flights$origin)

# Show factor levels

levels(flights$origin_factor)
## [1] "EWR" "JFK" "LGA"
# Frequency table

origin_table <- table(flights$origin_factor)
origin_table
## 
##    EWR    JFK    LGA 
## 120835 111279 104662
# Bar plot

barplot(origin_table,
main = "Number of Flights by Origin Airport",
ylab = "Number of Flights",
col = c("lightblue", "lightgreen", "salmon"))

Question 5: Recoding Factors

Recode origin_factor from Question 4 into a new column origin_recoded with full names: “JFK” to “Kennedy”, “LGA” to “LaGuardia”, “EWR” to “Newark” using fct_recode() or base R. Create a bar plot of the recoded factor.

Output: The new levels and bar plot.

# Recode using forcats::fct_recode

flights$origin_recoded <- fct_recode(flights$origin_factor,
"Kennedy"   = "JFK",
"LaGuardia" = "LGA",
"Newark"    = "EWR")

# New levels and frequency

levels(flights$origin_recoded)
## [1] "Newark"    "Kennedy"   "LaGuardia"
table(flights$origin_recoded)
## 
##    Newark   Kennedy LaGuardia 
##    120835    111279    104662
# Bar plot of recoded factor

barplot(table(flights$origin_recoded),
main = "Flights by Airport (Full Names)",
ylab = "Number of Flights",
col = c("#C6E2FF", "#D6F5E3", "#FFD8E0"))

Question 6: Handling Missing Data

Count missing values in dep_delay and arr_delay using colSums(is.na(flights)). Impute missing dep_delay values with 0 (assuming no delay for cancelled flights) in a new column dep_delay_imputed. Create a frequency table of dep_delay_imputed for delays between -20 and 20 minutes (use filter() to subset).

Output: NA counts, and the frequency table for imputed delays.

# Count missing values for dep_delay and arr_delay

na_counts <- colSums(is.na(flights[, c("dep_delay", "arr_delay")]))
na_counts  # Output: show counts of NAs
## dep_delay arr_delay 
##      8255      9430
# Impute missing dep_delay with 0 (new column)

flights <- flights %>%
mutate(dep_delay_imputed = ifelse(is.na(dep_delay), 0, dep_delay))

# Filter to delays between -20 and 20 (inclusive) and show frequency

delay_range <- flights %>%
filter(!is.na(dep_delay_imputed)) %>%
filter(dep_delay_imputed >= -20 & dep_delay_imputed <= 20)

table(delay_range$dep_delay_imputed)
## 
##   -20   -19   -18   -17   -16   -15   -14   -13   -12   -11   -10    -9    -8 
##    37    19    81   110   162   408   498   901  1594  2727  5891  7875 11791 
##    -7    -6    -5    -4    -3    -2    -1     0     1     2     3     4     5 
## 16752 20701 24821 24619 24218 21516 18813 24769  8050  6233  5450  4807  4447 
##     6     7     8     9    10    11    12    13    14    15    16    17    18 
##  3789  3520  3381  3062  2859  2756  2494  2414  2256  2140  2085  1873  1749 
##    19    20 
##  1730  1704

Question 7: Reflection (No Coding)

Reflect on the assignment: What was easy or hard about working with flight dates or missing data? How might assuming zero delay for missing values (Question 6) affect conclusions about flight punctuality? What did you learn about NYC flights in 2013? (150-200 words) I found creating datetime objects with lubridate helpful because it made combining separate year/month/day/time columns straightforward. Converting times like 517 into 5:17 was a little confusing at first but using integer division and modulus fixed that. Working with missing values was challenging because imputing zeros for cancelled flights may bias the results toward appearing more on-time. For future steps, I would explore imputing based on typical delays for similar flights or exclude cancelled flights depending on the research question. Overall, I learned more about how frequent delays were in NYC in 2013 and how date/time parsing is important for time series analysis.