Frequency polygon
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Frequency polygon is another way to show the information in a frequency table. It looks a little bit like a line graph. To make a frequencypolygon, you just need to plot a few points and then join the points by straight lines.Frequencypolygon of a given frequency distribution can be drawn in two ways.
1) By using histogram 2) Without using histogram
After drawing a histogram, obtain the midpoints of each class. The midpoint of a class is the point in the middle of the class. So for instance, if I have a class “10 – 19", then the midpoint is ( 10+19)/2 = 14.5. A class of “0 – 5" has a midpoint of 2.5. Join these mid points of the adjacent rectangles of the histogram by dotted line segments. Obtain the mid points of two class intervals of zero frequency on Xaxis, one adjacent to the first, on its left and one adjacent to the last, on its right. These classintervals are known as imagined class intervals.
Note : Sometimes imagined class intervals do not exist. For example marks obtained by the students in a test, we can not go below zero and beyond maximum marks on the two sides. In such cases extreme linesegments are partly drawn and are brought down vertically so that they meet with the vertical sides of first and the last rectangles.
Example without using histogram :


















10 12  










Note: Class marks are obtained by adding the age in years and divide it by 2.
Graph Dictionary
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