7 Important Ways to Summarise Data in R

Introduction

People remain confused when it comes to summarizing data real quick in R. There are various options. But, which one is the best ? I’ve answered this question below. You must choose one at first. And, become expert at it. That’s how you should move to the next.

People who transition from SAS or SQL are used to writing simple queries on these languages to summarize data sets. For such audience, the biggest concern is to how do we do the same thing on R.

In this article I will cover primary ways to summarize data sets. Hopefully this will make your journey much easier than it looks like.

Generally, summarizing data means finding statistical figures such as mean, median, box plot etc. If understand well with scatter plots & histogram, you can refer to guide on data visualization in R.

Methods to Summarise Data in R

1. apply

Apply function returns a vector or array or list of values obtained by applying a function to either rows or columns. This is the simplest of all the function which can do this job. However this function is very specific to collapsing either row or column.

m <- matrix(c(1:10, 11:20), nrow = 10, ncol = 2)
apply(m, 1, mean)
[1]  6  7  8  9 10 11 12 13 14 15

apply(m, 2, mean)
[1]  5.5 15.5

2. lapply

“lapply” returns a list of the same length as X, each element of which is the result of applying FUN to the corresponding element of X.”

l <- list(a = 1:10, b = 11:20)
lapply(l, mean)
$a
[1] 5.5

$b
[1] 15.5

3.  sapply

“sapply” does the same thing as apply but returns a vector or matrix. Let’s consider the last example again.

l <- list(a = 1:10, b = 11:20) l.mean <- sapply(l, mean)
class(l.mean)
[1] "numeric"

4. tapply

Till now, all the function we discussed cannot do what Sql can achieve. Here is a function which completes the palette for R. Usage is “tapply(X, INDEX, FUN = NULL, …, simplify = TRUE)”, where X is “an atomic object, typically a vector” and INDEX is “a list of factors, each of same length as X”. Here is an example which will make the usage clear.

attach(iris)
# mean petal length by species
tapply(iris$Petal.Length, Species, mean)
    setosa versicolor  virginica 
     1.462      4.260      5.552

attach(iris)
by(iris[, 1:4], Species, colMeans)

Species: setosa
Sepal.Length  Sepal.Width Petal.Length  Petal.Width 
       5.006        3.428        1.462        0.246 

------------------------------------------------------------ 

Species: versicolor
Sepal.Length  Sepal.Width Petal.Length  Petal.Width 
       5.936        2.770        4.260        1.326 

------------------------------------------------------------ 

Species: virginica
Sepal.Length  Sepal.Width Petal.Length  Petal.Width
       6.588        2.974        5.552        2.026

attach(iris)
summarization <- sqldf(select Species, mean(Petal.Length) from Petal.Length_mean where Species is not null group by Species’)

7. ddply

library(plyr)
attach(iris)

# mean petal length by species

ddply(iris,"Species",summarise, Petal.Length_mean = mean (Petal.Length))

Additional Notes: You can also use packages such as dplyr, data.table to summarize data.  Here’s a complete tutorial on useful packages for data manipulation in R – Faster Data Manipulation with these 7 R Packages.

In general if you are trying to add this summarisation step in the middle of a process and need a table as output, you need to go for sqldf or ddply. “ddply” in these cases is faster but will not give you options beyond just grouping. “sqldf” has all features you need to summarize the data in SQL statements.

In case you are interested in using function similar to pivot tables or transposing the tables, you can consider using “reshape”. We have covered a few examples of the same in our article – comprehensive guide for data exploration in R.

Challenge : Here is a simple problem you can attempt to solve using all the methods we have discussed. You have a table for all school kids marks in a particular city.

Write a code to find the mean marks of each school for both class 1 and 2, for students with roll no less than 6. And print only the class whose mean score comes out to be higher for the school. For instance, if school A has a mean score of 6 for class 1 and 4 for class 2, you will reject class 2 and only take class 1 mean score for the school. In cases of tie, you can make a random choice. Assume that the actual table is much bigger and keep the code as generalized as possible.

End Notes