NCERT Book Class 10 Maths Chapter 13 Statistics

STATISTICS 171

STATISTICS 13
13.1 Introduction
In Class IX, you have studied the classification of given data into ungrouped as well as
grouped frequency distributions. You have also learnt to represent the data pictorially
in the form of various graphs such as bar graphs, histograms (including those of varying
widths) and frequency polygons. In fact, you went a step further by studying certain
numerical representatives of the ungrouped data, also called measures of central
tendency, namely, mean, median and mode. In this chapter, we shall extend the study
of these three measures, i.e., mean, median and mode from ungrouped data to that of
grouped data. We shall also discuss the concept of cumulative frequency, the
cumulative frequency distribution and how to draw cumulative frequency curves, called
ogives.

13.2 Mean of Grouped Data
The mean (or average) of observations, as we know, is the sum of the values of all the
observations divided by the total number of observations. From Class IX, recall that if
x1, x2,. . ., xn are observations with respective frequencies f1, f2, . . ., fn, then this
means observation x1 occurs f1 times, x2 occurs f2 times, and so on.
Now, the sum of the values of all the observations = f1x1 + f2x2 + . . . + fnxn, and
the number of observations = f1 + f2 + . . . + fn.
So, the mean x of the data is given by
f x  f 2 x2    f n xn
x = 1 1
f1  f 2    f n
Recall that we can write this in short form by using the Greek letter  (capital
sigma) which means summation. That is,