R Statistical Software Basics – Descriptive Statistics - Percentiles

The practice sheet titled StatisticMarks Data.csv downloaded from Link. – Download Sheet

About the Data Sheet - The data in this sheet is related to marks scored by 100 Students in a
Statistical Test.

Based on the data, we will use R Software Statistical functions to analyze the descriptive statistics.

In the Data Sheet, we have Data from A2:A101, A1 being the header of the Data. I have stored the StatisticMarks.csv file in Working Directory on my Desktop.

setwd("C:/Users/Rajesh Prabhakar/Desktop/R")

For inputting or reading Data from “StatisticMarks.csv” file, R Command would be

StatMarks=read.csv("StatisticMarks.csv")

Percentiles

Assume that the elements in a data set are rank ordered from the smallest to the largest. The values that divide a rank-ordered set of elements into 100 equal parts are called percentiles.

An element having a percentile rank of P_i would have a greater value than i percent of all the elements in the set. Thus, the observation at the 50th percentile would be denoted P₅₀, and it would be greater than 50 percent of the observations in the set.

An observation at the 50th percentile would correspond to the median value in the set.

In R Statistical Software, Quartiles & percentiles are represented by function called “quantile”

quantile(filename, c(.10,.20,.30,.40,.50,.60,.70,.80,.90,.95))

In the Data Sheet, we have Data from A2:A101, A1 being the header of the Data titled StatisticsMarks.

quantile(StatMarks$StatisticsMarks, c(.10,.20,.30,.40,.50,.60,.70,.80,.90,.95)) 

StatMarks is the name of the variable in which we stored the data followed by $ sign and column header of the Data i.e. StatisticsMarks.

Remember the title of the column should be exactly same including the large caps & small caps or else it will give error.

In R the file names, column headers and row headers should exactly match the same or else the function will give errors

The result of this function in R Console is

> quantile(StatMarks$StatisticsMarks,c(.10,.20,.30,.40,.50,.60,.70,.80,.90,.95))

 10%  20%  30%  40%  50%  60%  70%  80%  90%  95%

  56.8 66.0   71.0  73.0   75.0  78.0  80.0   81.2  91.1   97.0

10% of students scored upto 56.8 marks, 20% of students scored upto 66 marks, 30% scored upto 71 marks, 50% scored upto 75 marks, etc.