Page 128 - Ganit Kaushal

Summary of the Chapter

• In various fields, we need information in the form of numerical figures. Each figure of this kind is called
an observation. The collection of all the observations is called data.
• A collection of observations gathered initially is called raw data.

• The difference between the highest and the lowest values of the observations in a data set is called the
range of the data.
• The data collected directly from the source for the first time by the observer is called primary data.
• When the data is collected from any external source, i.e., from newspapers, magazines, TV, internet,
etc., it is called secondary data.

• When the number of observations is large, to minimise the number of errors and to make tabulation
easier, we can use tally marks. In the tally marks method, we draw three columns. In the first column,
we write the number (variable), in the second column (tally marks), e.g. if a particular number occurs
more than 4 times, we represent it by cutting the first four tally marks diagonally, this gives us a block of
five ( ||| = 8, |||| = 9, etc.). Tally marks are always recorded in bunches of five. Finally, after counting
tally marks, the actual numbers can be written in the third column, which represents frequency.

• Frequency is a number that tells us how many times a particular observation in a given set of data occurs.
• A pictograph is a way of representing data through pictures of objects. A pictograph uses picture
symbols to convey meaningful information. Each picture symbol represents a certain number of things. A
pictogram or pictograph represents the frequency of data as pictures or symbols. Each picture or symbol
may represent one or more units of the data.
• A bar graph is a graph that presents categorical data with rectangular bars with heights or lengths
proportional to the values that they represent. The bars can be plotted vertically or horizontally. Bar
graphs can be used to show how something changes over time or to compare items. A typical bar graph

will have a labelled axis, scales and bars. Bar graphs have a labelled x-axis (horizontal axis) and y-axis
(vertical axis).
These are the features of the bar graph:

(i) It has rectangular bars of equal width.
(ii) The space between two consecutive bars must be the same.
(iii) Bars can be marked both vertically and horizontally, but normally we use vertical bars.

(iv) The height of the bar represents the frequency of corresponding observations.
The important thing to know is that the longer the bar, the greater is its value.

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