Frequency Polygon

The Frequency Polygon is another visual way of representing quantitative data and its frequencies. This is a polygon. To see what we mean, consider the histogram represented below. Let us join the mid-points of the upper sides of the adjacent rectangles of this histogram by means of line segments. Let us call these mid-points B, C, D, E, F and G. When joined by line segments, we obtain the figure BCDEFG.

To complete the polygon, we assume that there is a class interval with frequency zero before 30.5 - 35.5, and one after 55.5 - 60.5, and their mid-points are A and H, respectively. ABCDEFGH is the frequency polygon corresponding to the data shown in the figure.

Although, there exists no class preceding the lowest class and no class succeeding the highest class, addition of the two class intervals with zero frequency enables us to make the area of the frequency polygon the same as the area of the histogram. Why is this so? (Hint : Use the properties of congruent triangles.)

Now, the question arises: how do we complete the polygon when there is no class preceding the first class? Let us consider such a situation.

Example 4 : Consider the marks, out of 100, obtained by 51 students of a class in a test, given in Table

Draw a frequency polygon corresponding to this frequency distribution table.

Solution : Let us first draw a histogram for this data and mark the mid-pointspoints of the tops of the rectangles as B, C, D, E, F, G, H, I, J, K respectively.

Continue

Here, the first class is 0-10. So, to find the class preceeding 0-10, we extend the horizontal axis in the negative direction and find the midpointline segment of the imaginary class-interval (–10) - 0. The first end point, i.e., B is joined to this mid-point with zero frequency on the negative direction of the horizontal axis. The pointmid-point where this line segment meets the vertical axis is marked as A. Let L be the mid-point of the class succeeding the lastmiddlefirst class of the given data.

Then the required frequencycontinous polygon is OABCDEFGHIJKL, which is shown in above Fig.

Frequency polygons can also be drawn independently without drawing histograms. For this, we require the mid-points of the class-intervals used in the data.

These mid-points of the class-intervals are called class-marks class-points.

To find the class-mark of a class interval, we find the sum of the upper limit and lower limit of a class and divide it by 2. Thus,

Let us consider an example.

Example 5 : In a city, the weekly observations made in a study on the cost of living index are given in the following table:

Draw a frequency polygon for the data above (without constructing a histogram). Solution : Since we want to draw a frequency polygon without a histogram, let us find the class-marks of the classes given above, that is of 140 - 150, 150 - 160,.... For 140 - 150, the upper limit = 150, and the lower limit = 140

So, the class-mark = 150+1402 = 2902 = .

Continuing in the same manner, we find the class-marks of the other classes as well. So, the new table obtained is as shown in the following table:

We can now draw a frequency polygon by plotting the class-marks along the horizontal axis, the frequencies along the vertical-axis, and then plotting and joining the points B(145, 5), C(155, 10), D(165, 20), E(175, 9), F(185, 6) and G(195, 2) by line segments. We should not forget to plot the point corresponding to the class-mark of the class 130 - 140 (just before the lowest class 140 - 150) with zero frequency, that is, A(135, 0), and the point H (205, 0) occurs immediately after G(195, 2). So, the resultant frequency polygon will be ABCDEFGH (see Fig. 12.8)

Graph Figure

Frequency polygons are used when the data is continuous and very largevery small. It is very useful for comparing two different sets of data of the samedifferent nature, for example, comparing the performance of two different sections of the same class.