Building a Price Index from a Market Basket

Tracking every price in an economy is impossible and pointless — a bushel of wheat and a laptop don't move together, and no one buys just one of anything. So instead, statisticians fix a market basket: a representative bundle of goods and services households actually buy, in fixed quantities. Because the basket never changes, any change in its total cost comes from price changes alone, not from people buying more or less stuff. That isolation is the whole point.

To turn that basket into a usable number, pick a base year and call its basket cost the benchmark, assigned a value of 100. Then for any other year, the formula is: CPI = (cost of basket in current year / cost of basket in base year) x 100. The base year's own CPI is always 100 by definition. Every other year's CPI tells you, in one glance, how the price level compares to that benchmark — no unit conversions, no guessing.

This index feeds directly into the inflation rate, which is just the percent change in CPI from one year to the next, and into real versus nominal comparisons elsewhere in the unit. Get the index right and inflation, real income, and real GDP calculations all follow cleanly; get the base-year logic wrong and every downstream number is misread.