Construct a frequency polygon from the following data :

Class-interval	Frequency
1 - 5	5
6 - 10	8
11 - 15	12
16 - 20	7
21 - 25	4

The following frequency distribution table is discontinuous. Convert it into continuous frequency distribution.

Adjustment factor = $\dfrac{\text{Lower limit of one class - Upper limit of previous class}}{2}$

= $\dfrac{6 - 5}{2} = \dfrac{1}{2}$ = 0.5.

Subtract the adjustment factor (0.5) from all the lower limits and add the adjustment factor (0.5) to all the upper limits.

Continuous frequency distribution table :

Steps to draw frequency polygon :

1. Take 2 cm along x-axis = 5 units.

2. Take 2 cm along y-axis = 2 units.

3. Find the mid-point of class intervals.

4. A kink is drawn near x-axis to show that the scale begins at 0.5.

5. Find points corresponding to given frequencies of classes and the mid-points of class-intervals, and plot them.

6. Join consecutive points by line segments.

7. Join first end point with mid-point of class -5.5 - 0.5 with zero frequency and join the other end with mid-point of class 25.5 - 30.5 with zero frequency.

The required frequency polygon is shown below: