Random Sampling a Dataset in R

A common example in business  analytics data is to take a random sample of a very large dataset, to test your analytics code. Note most business analytics datasets are data.frame ( records as rows and variables as columns)  in structure or database bound.This is partly due to a legacy of traditional analytics software.

Here is how we do it in R-

• Refering to parts of data.frame rather than whole dataset.

Using square brackets to reference variable columns and rows

The notation dataset[i,k] refers to element in the ith row and jth column.

The notation dataset[i,] refers to all elements in the ith row .or a record for a data.frame

The notation dataset[,j] refers to all elements in the jth column- or a variable for a data.frame.

For a data.frame dataset

> nrow(dataset) #This gives number of rows

> ncol(dataset) #This gives number of columns

An example for corelation between only a few variables in a data.frame.

> cor(dataset1[,4:6])

Splitting a dataset into test and control.

ts.test=dataset2[1:200] #First 200 rows

ts.control=dataset2[201:275] #Next 75 rows

• Sampling

Random sampling enables us to work on a smaller size of the whole dataset.

use sample to create a random permutation of the vector x.

Suppose we want to take a 5% sample of a data frame with no replacement.

Let us create a dataset ajay of random numbers

ajay=matrix( round(rnorm(200, 5,15)), ncol=10)

#This is the kind of code line that frightens most MBAs!!

Note we use the round function to round off values.

ajay=as.data.frame(ajay)

 nrow(ajay)

[1] 20

> ncol(ajay)

[1] 10

This is a typical business data scenario when we want to select only a few records to do our analysis (or test our code), but have all the columns for those records. Let  us assume we want to sample only 5% of the whole data so we can run our code on it

Then the number of rows in the new object will be 0.05*nrow(ajay).That will be the size of the sample.

The new object can be referenced to choose only a sample of all rows in original object using the size parameter.

We also use the replace=FALSE or F , to not the same row again and again. The new_rows is thus a 5% sample of the existing rows.

Then using the square backets and ajay[new_rows,] to get-

b=ajay[sample(nrow(ajay),replace=F,size=0.05*nrow(ajay)),]

 

You can change the percentage from 5 % to whatever you want accordingly.

Author: Ajay Ohri

http://about.me/ajayohri

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