EDA Missing Values

suppressPackageStartupMessages(library("tidyverse"))

package 㤼㸱tidyverse㤼㸲 was built under R version 3.6.3

1. What happens to missing values in a histogram? What happens to missing values in a bar chart? Why is there a difference?

Missing values are removed when the number of observations in each bin are calculated. See the warning message: Removed 9 rows containing non-finite values (stat_bin)

diamonds2 <- diamonds %>%
  mutate(y = ifelse(y < 3 | y > 20, NA, y))

ggplot(diamonds2, aes(x = y)) +
  geom_histogram()

In the geom_bar() function, NA is treated as another category. The x aesthetic in geom_bar() requires a discrete (categorical) variable, and missing values act like another category.

diamonds %>%
  mutate(cut = if_else(runif(n()) < 0.1, NA_character_, as.character(cut))) %>%
  ggplot() +
  geom_bar(mapping = aes(x = cut))

In a histogram, the x aesthetic variable needs to be numeric, and stat_bin() groups the observations by ranges into bins. Since the numeric value of the NA observations is unknown, they cannot be placed in a particular bin, and are dropped.

2. What does na.rm = TRUE do in mean() and sum()?

This option removes NA values from the vector prior to calculating the mean and sum.

mean(c(0, 1, 2, NA), na.rm = TRUE)

[1] 1

sum(c(0, 1, 2, NA), na.rm = TRUE)

[1] 3