Thursday 31 January 2019

Random forest in R Language

In the random forest approach, a large number of decision trees are created. Every observation is fed into every decision tree. The most common outcome for each observation is used as the final output. A new observation is fed into all the trees and taking a majority vote for each classification model.
An error estimate is made for the cases which were not used while building the tree. That is called an OOB (Out-of-bag) error estimate which is mentioned as a percentage.
The R package "randomForest" is used to create random forests.
Install R Package
Use the below command in R console to install the package. You also have to install the dependent packages if any.
The package "randomForest" has the function randomForest() which is used to create and analyze random forests.

The basic syntax for creating a random forest in R is −
randomForest(formula, data)
Following is the description of the parameters used −
  • formula is a formula describing the predictor and response variables.
  • data is the name of the data set used.

We will use the R in-built data set named readingSkills to create a decision tree. It describes the score of someone's readingSkills if we know the variables "age","shoesize","score" and whether the person is a native speaker.
Here is the sample data.
# Load the party package. It will automatically load other required packages.

# Print some records from data set readingSkills.
When we execute the above code, it produces the following result and chart −
  nativeSpeaker   age   shoeSize      score
1           yes     5   24.83189   32.29385
2           yes     6   25.95238   36.63105
3            no    11   30.42170   49.60593
4           yes     7   28.66450   40.28456
5           yes    11   31.88207   55.46085
6           yes    10   30.07843   52.83124
Loading required package: methods
Loading required package: grid

We will use the randomForest() function to create the decision tree and see it's graph.
# Load the party package. It will automatically load other required packages.

# Create the forest.
output.forest <- randomForest(nativeSpeaker ~ age + shoeSize + score, 
           data = readingSkills)

# View the forest results.

# Importance of each predictor.
print(importance(fit,type = 2)) 
When we execute the above code, it produces the following result −
 randomForest(formula = nativeSpeaker ~ age + shoeSize + score,     
                 data = readingSkills)
               Type of random forest: classification
                     Number of trees: 500
No. of variables tried at each split: 1

        OOB estimate of  error rate: 1%
Confusion matrix:
    no yes class.error
no  99   1        0.01
yes  1  99        0.01
age              13.95406
shoeSize         18.91006
score            56.73051

From the random forest shown above we can conclude that the shoesize and score are the important factors deciding if someone is a native speaker or not. Also the model has only 1% error which means we can predict with 99% accuracy.


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