Index number
Topic: Geographical Data: Sources, Compilation and Processing · NCERT: Class 12, Ch 1 "Data - Its Source and Compilation"
Meaning
An index number shows how much a value has changed compared with a base value, which is set at 100. A figure above 100 means a rise and a figure below 100 means a fall. In the simple aggregate method, you divide the current total by the base total and multiply by 100: Σq₁ ÷ Σq₀ × 100. Index numbers can compare different times, places, industries or countries.
Example
India produced 32.5 million tonnes of iron ore in 1970-71, which is taken as the base (= 100). Output rose to 67.4 million tonnes in 2000-01. So the index is 67.4 ÷ 32.5 × 100 ≈ 207. This means output roughly doubled.
Don't confuse with
- Percentage data: a percentage shows a part out of 100 of a whole at one time. An index number shows change against a base value.