slides.Rmd

title: "R#5: data transformation"
author: "Laurent Modolo [laurent.modolo@ens-lyon.fr](mailto:laurent.modolo@ens-lyon.fr)"
date: "28 Nov 2019"
output:
  slidy_presentation:
    highlight: tango
  beamer_presentation:
    theme: metropolis
    slide_level: 3
    fig_caption: no
    df_print: tibble
    highlight: tango
    latex_engine: xelatex

knitr::opts_chunk$set(echo = FALSE)
library(tidyverse)

Grouped summaries with summarise()

summarise() collapses a data frame to a single row:

library(nycflights13)
library(tidyverse)
flights %>% 
  summarise(delay = mean(dep_delay, na.rm = TRUE))

The power of summarise() with group_by()

This changes the unit of analysis from the complete dataset to individual groups. Then, when you use the dplyr verbs on a grouped data frame they’ll be automatically applied “by group”.

flights %>% 
  group_by(year, month, day) %>% 
  summarise(delay = mean(dep_delay, na.rm = TRUE))

5_a

Challenge with summarise() and group_by()

Imagine that we want to explore the relationship between the distance and average delay for each location. here are three steps to prepare this data:

• Group flights by destination.
• Summarise to compute distance, average delay, and number of flights.
• Filter to remove noisy points and Honolulu airport, which is almost twice as far away as the next closest airport.

flights %>% 
  group_by(dest)

Challenge with summarise() and group_by()

Imagine that we want to explore the relationship between the distance and average delay for each location.

• Filter to remove noisy points and Honolulu airport, which is almost twice as far away as the next closest airport.

flights %>% 
  group_by(dest) %>% 
  summarise(
    count = n(),
    dist = mean(distance, na.rm = TRUE),
    delay = mean(arr_delay, na.rm = TRUE)
  )

Missing values

You may have wondered about the na.rm argument we used above. What happens if we don’t set it?

flights %>% 
  group_by(dest) %>% 
  summarise(
    dist = mean(distance),
    delay = mean(arr_delay)
  )

Aggregation functions obey the usual rule of missing values: if there’s any missing value in the input, the output will be a missing value.

Counts

Whenever you do any aggregation, it’s always a good idea to include either a count (n()). That way you can check that you’re not drawing conclusions based on very small amounts of data.

flights %>% 
  group_by(dest) %>% 
  summarise(
    count = n(),
    dist = mean(distance, na.rm = TRUE),
    delay = mean(arr_delay, na.rm = TRUE)
  )

Challenge with summarise() and group_by()

Imagine that we want to explore the relationship between the distance and average delay for each location.

• Summarise to compute distance, average delay, and number of flights.
• Filter to remove noisy points and Honolulu airport, which is almost twice as far away as the next closest airport.

flights %>% 
  group_by(dest) %>% 
  summarise(
    count = n(),
    dist = mean(distance, na.rm = TRUE),
    delay = mean(arr_delay, na.rm = TRUE)
  ) %>% 
  filter(count > 20, dest != "HNL")

Challenge with summarise() and group_by()

Imagine that we want to explore the relationship between the distance and average delay for each location.

flights %>% 
  group_by(dest) %>% 
  summarise(
    count = n(),
    dist = mean(distance, na.rm = TRUE),
    delay = mean(arr_delay, na.rm = TRUE)
  ) %>% 
  filter(count > 20, dest != "HNL") %>% 
  ggplot(mapping = aes(x = dist, y = delay)) +
  geom_point(aes(size = count), alpha = 1/3) +
  geom_smooth(se = FALSE)

5_b

Challenge with summarise() and group_by()

flights %>% 
  group_by(dest) %>% 
  summarise(
    count = n(),
    dist = mean(distance, na.rm = TRUE),
    delay = mean(arr_delay, na.rm = TRUE)
  ) %>% 
  filter(count > 20, dest != "HNL") %>% 
  ggplot(mapping = aes(x = dist, y = delay)) +
  geom_point(aes(size = count), alpha = 1/3) +
  geom_smooth(se = FALSE)

Ungrouping

If you need to remove grouping, and return to operations on ungrouped data, use ungroup().

flights %>% 
  group_by(year, month, day) %>% 
  ungroup() %>%
  summarise(delay = mean(dep_delay, na.rm = TRUE))

Grouping challenges

• Look at the number of cancelled flights per day. Is there a pattern? Is the proportion of cancelled flights related to the average delay? (strftime(x,'%A') give you the name of the day from a POSIXct date)
• Which carrier has the worst delays? Challenge: can you disentangle the effects of bad airports vs. bad carriers? Why/why not? (Hint: think about flights %>% group_by(carrier, dest) %>% summarise(n()))

Grouping challenges

• Look at the number of cancelled flights per day. Is there a pattern? Is the proportion of cancelled flights related to the average delay? (strftime(x,'%A') give you the name of the day from a POSIXct date)

flights %>% 
  mutate(
    canceled = is.na(dep_time) | is.na(arr_time)
  ) %>% 
  filter(canceled) %>% 
  mutate(wday = strftime(time_hour,'%A')) %>% 
  group_by(wday) %>% 
  summarise(
    cancel_day = n()
  ) %>%
  ggplot(mapping = aes(x = fct_reorder(wday, cancel_day), y = cancel_day)) +
  geom_col()

5_b

Grouping challenges

• Look at the number of cancelled flights per day. Is there a pattern? Is the proportion of cancelled flights related to the average delay? (strftime(x,'%A') give you the name of the day from a POSIXct date)

flights %>% 
  mutate(
    canceled = is.na(dep_time) | is.na(arr_time)
  ) %>% 
  mutate(wday = strftime(time_hour,'%A')) %>% 
  group_by(wday) %>% 
  summarise(
    cancel_day = n()
  ) %>%
  ggplot(mapping = aes(x = wday, y = cancel_day)) +
  geom_col()

Grouping challenges

• Which carrier has the worst delays?

flights %>% 
  group_by(carrier) %>% 
  summarise(
    carrier_delay = mean(arr_delay, na.rm = T)
  ) %>%
  mutate(carrier = fct_reorder(carrier, carrier_delay)) %>%
  ggplot(mapping = aes(x = carrier, y = carrier_delay)) +
  geom_col(alpha = 0.5)

5_c

Grouping challenges

• Which carrier has the worst delays?

flights %>% 
  group_by(carrier) %>% 
  summarise(
    carrier_delay = mean(arr_delay, na.rm = T)
  ) %>%
  mutate(carrier = fct_reorder(carrier, carrier_delay)) %>%
  ggplot(mapping = aes(x = carrier, y = carrier_delay)) +
  geom_col(alpha = 0.5)

Grouped mutates (and filters)

Grouping is also useful in conjunction with mutate() and filter()

• Find all groups bigger than a threshold:
• Standardise to compute per group metrics:

flights %>% 
  group_by(dest, year) %>% 
  filter(n() > 10000) %>% 
  filter(arr_delay > 0) %>% 
  mutate(prop_delay = arr_delay / sum(arr_delay)) %>% 
  select(year:day, dest, arr_delay, prop_delay)

Goup by challenges

• What time of day should you fly if you want to avoid delays as much as possible?

flights %>% 
  group_by(hour) %>% 
  summarise(
    mean_delay = mean(arr_delay, na.rm = T),
    sd_delay = sd(arr_delay, na.rm = T),
  ) %>% 
  ggplot() +
  geom_errorbar(mapping = aes(
    x = hour,
    ymax = mean_delay + sd_delay,
    ymin = mean_delay - sd_delay)) +
  geom_point(mapping = aes(
    x = hour,
    y = mean_delay,
  ))

5_d