Frequency density
Topic: Economic Data: Census, NSS, Surveys and Statistical Tools · NCERT: Class 11, Ch 4 "Presentation of Data"
Meaning
Frequency density is the number of observations for each unit of class width. It is used when the classes in a table have different widths. Frequency density = class frequency ÷ class width In a histogram, the area of each rectangle must match its frequency. If the classes have different widths, the height of each rectangle must be the frequency density, not the plain frequency. Otherwise a wide class looks bigger than it really is.
Example
In NCERT's table of marks for 100 students, the class 40-50 has 21 students. Its width is 10, so its frequency density is 2.1 students per mark. Suppose we merge 60-70 (19 students) and 70-80 (6 students) into one class, 60-80. It now has 25 students over a width of 20, so its density is only 1.25. If we used the frequency of 25 as the height, this wide class would wrongly look taller than 40-50.
Don't confuse with
- Relative frequency: this is class frequency ÷ total frequency, a share of the whole. Frequency density divides by the class width.
Related concepts
- Raw data
- Classification
- Chronological classification
- Spatial classification
- Qualitative classification
- Quantitative classification
- Attribute
- Continuous variable
- Discrete variable
- Frequency