You can use the aggregate() function in R to calculate summary statistics for variables in a data frame.
By default, if the aggregate() function encounters a row in a data frame with one or more NA values, it will simply drop the row when performing calculations.
This can cause unintended consequences when performing calculations.
To avoid this behavior, you can use the argument na.action=NULL within the aggregate() function.
The following example shows how to use this argument in practice.
Example: Use aggegate() in R and Do Not Drop Rows with NA
Suppose we have the following data frame in R that shows the points and assists for basketball players on various teams:
#create data frame df <- data.frame(team=c('A', 'A', 'A', 'A', 'A', 'B', 'B', 'B', 'C', 'C'), points=c(5, 9, 12, 14, 14, 13, 10, 6, 15, 18), assists=c(NA, 4, 4, 5, 3, 6, 8, 4, 10, NA)) #view data frame df team points assists 1 A 5 NA 2 A 9 4 3 A 12 4 4 A 14 5 5 A 14 3 6 B 13 6 7 B 10 8 8 B 6 4 9 C 15 10 10 C 18 NA
Now suppose that we attempt to use the aggregate() function to calculate the sum of points and assists, grouped by team:
#attempt to calculate sum of points and assists, grouped by team aggregate(. ~ team, data=df, FUN=sum, na.rm=TRUE) team points assists 1 A 49 16 2 B 29 18 3 C 15 10
The output appears to show us the sum of points and assists by team, but the rows with NA values were actually dropped when performing these calculations.
We can confirm this by viewing the original data frame and seeing that team C has two values in the points column:
Thus, team C should have a sum of points of 33, but the output only shows 15.
This is because the row with a points value of 18 has a value of NA in the assists column, which means this row was actually not used when calculating the sum of points for team C.
To ensure that rows with NA values are not dropped when performing calculations, we must use the argument na.action=NULL as follows:
#calculate sum of points and assists, grouped by team (don't drop NA rows) aggregate(. ~ team, data=df, FUN=sum, na.rm=TRUE, na.action=NULL) team points assists 1 A 54 16 2 B 29 18 3 C 33 10
Notice that the aggregate() function correctly returns a sum of points values for team C as 33 this time.
Note: The argument na.rm=TRUE specifies that NA values should be ignored when performing a calculation in a specific column.
The following tutorials explain how to perform other common tasks in R:
How to Use summary() Function in R
How to Calculate the Mean by Group in R
How to Calculate Descriptive Statistics in R