Frequency polygon
Topic: Economic Data: Census, NSS, Surveys and Statistical Tools · NCERT: Class 11, Ch 4 "Presentation of Data"
Meaning
A frequency polygon is a line graph of a frequency distribution. To draw it, first draw the histogram. Then join the midpoints of the tops of its rectangles with straight lines. Finally, close the figure at both ends on the base line, at the midpoints of the zero-frequency classes just before the first class and just after the last class. It can also be drawn without a histogram, by plotting each class mark against its frequency. Several polygons can sit on one graph without hiding each other, so it is the best diagram for comparing two or more distributions.
Example
Plot NCERT's marks of 100 students at class marks 5, 15, 25 … 95 with frequencies 1, 8, 6 … 4, and join the points. The line peaks at 55, the middle of the 50-60 class, which has 23 students. You could plot a second class's marks on the same graph to compare them.
Don't confuse with
- Frequency curve: this is a smooth freehand curve drawn through the same points, not straight line pieces.
Related concepts
- Raw data
- Classification
- Chronological classification
- Spatial classification
- Qualitative classification
- Quantitative classification
- Attribute
- Continuous variable
- Discrete variable
- Frequency