What is the formula for buying power index?

The Buying Power Index (BPI) is calculated using the formula: BPI = (Local Income / National Income) × (National Cost of Living / Local Cost of Living) × 100. This index helps to compare the purchasing power of different regions by adjusting for income levels and cost of living differences. A BPI above 100 indicates higher buying power relative to the national average, while a value below 100 indicates lower buying power.