Page 138 - DLIS401_METHODOLOGY_OF_RESEARCH_AND_STATISTICAL_TECHNIQUES

Unit 9: Presentation of Data

Pie Charts Notes

Sometimes a circle is used to represent a given data. The various parts of it are proportionally
represented by sectors of the circle. Then the graph is called a Pie Graph or Pie Chart.
Frequency Polygon
In a frequency distribution, the mid-value of each class is obtained. Then on the graph paper,
the frequency is plotted against the corresponding mid-value. These points are joined by
straight lines. These straight lines may be extended in both directions to meet the X - axis to
form a polygon.
Histogram
A two dimensional frequency density diagram is called a histogram. A histogram is a diagram
which represents the class interval and frequency in the form of a rectangle.

9.3 Types of Graphical Presentation of Data

Here only a few of the standard graphic forms of representing the data are being discussed
as listed below (Beriefly we have discussed above):
• Histogram
• Bar Diagram or Bar Graph

• Frequency Polygon
• Cumulative Frequency Curve or Ogive

9.3.1 Histogram
The most common form of graphical presentation of data is histogram. For plotting a histogram,
one has to take a graph paper. The values of the variable are taken on the horizontal axis/
scale known as X-axis and the frequencies are taken on the vertical axis/scale known as Y-
axis. For each class interval a rectangle is drawn with the base equal to the length of the
class interval and height according to the frequency of the C.I. When C.I. are of equal length,
which would generally be the case in the type of data you are likely to handle in school
situations, the heights of rectangles must be proportional to the frequencies of the Class
Intervals.
When the C.I. are not of equal length, the areas of rectangles must be proportional to the
frequencies indicated (most likely you will not face this type of situation). As the C.I.s for
any variable are in continuity, the base of the rectangles also extends from one boundary
to the other in continuity. These boundaries of the C.I.s are indicated on the horizontal
scale.
The frequencies for determining the heights of the rectangles are indicated on the vertical
scale of the graph.
Let us prepare a histogram for the frequency distribution of mathematics score of 120 Class
X students (Table 9.2).
For this, on the horizontal axis of the graph one has to mark the boundaries of the class
intervals, starting from the lowest, which is 34.5 to 39.5. So the points on X-axis will be 34.5,
39.5, 44.5, 49.5, ....... 99.5. Now on the vertical axis of the graph, the frequencies from 1 to 14
are to be marked. The height of the graphical presentation is usually taken as 60 to 75% of the
width. Here, we take 1 cm on X-axis representing 5 scores and 1 cm on Y-axis representing
a frequency of 2. For plotting the first rectangle, the base to be taken is 34.5–39.5 and the
height is 7, for the second the base is 39.5–44.5 and the height is 8, and so on.

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