Class mark
Also called: Class mid-point, Mid-value · Topic: Economic Data: Census, NSS, Surveys and Statistical Tools · NCERT: Class 11, Ch 3 "Organisation of Data"; Class 11, Ch 5 "Measures of Central Tendency"; Class 11, Ch 8 "Use of Statistical Tools"
Meaning
The class mark is the middle value of a class.
Class mark = (Upper class limit + Lower class limit) / 2
In later calculations, the class mark stands for every observation in that class. For grouped data, for example, the mean is worked out as X̄ = Σfm/Σf, where m is the class mark. This causes some loss of information, because the actual values are replaced by one value.
Example
For the class 20-30, the class mark is (30 + 20)/2 = 25. If the actual values in the class are 20, 22, 25, 25, 25 and 28, all of them are treated as 25 once the data are grouped.
Don't confuse with
- Class interval: the width of the class (upper minus lower limit). The class mark is its midpoint.
Related concepts
- Raw data
- Classification
- Chronological classification
- Spatial classification
- Qualitative classification
- Quantitative classification
- Attribute
- Continuous variable
- Discrete variable
- Frequency