Unequal class intervals
Topic: Economic Data: Census, NSS, Surveys and Statistical Tools · NCERT: Class 11, Ch 3 "Organisation of Data"; Class 11, Ch 4 "Presentation of Data"
Meaning
Unequal class intervals are classes of different widths in a frequency distribution. They are used in two cases:
- The range is very large. Income, for example, can run from near zero to crores.
- Most values cluster in a small part of the range. Narrow classes are then used there, and wide classes elsewhere.
They show detail where the data are dense. On a histogram, the height of each bar must then be the frequency density, which is class frequency ÷ class width. Using the raw frequency would give a false picture.
Example
In NCERT's marks table, most of the 100 students scored between 40 and 70. So that band is split into narrower classes of width 5, and the other classes stay wider. Age-at-death tables use very short intervals at early ages for the same reason.
Don't confuse with
- Equal class intervals: here every class has the same width. Frequency can then be plotted directly on a histogram.
Related concepts
- Raw data
- Classification
- Chronological classification
- Spatial classification
- Qualitative classification
- Quantitative classification
- Attribute
- Continuous variable
- Discrete variable
- Frequency