Index Numbers

The full picture

Imagine you want to know whether life has become more expensive over the last five years. Prices of hundreds of goods — rice, petrol, medicine, school fees — have all changed by different amounts. How do you combine all these into one clean answer? That is exactly what an index number does. It is a single number that measures the average percentage change in a group of related items from a base year, which is fixed at 100. If the index rises to 140, prices are on average 40% higher than they were in the base year.

To build an index number, you follow four steps. First, choose a basket of items — the goods and services that matter to the people you are measuring for. India's Consumer Price Index (CPI) basket includes food and beverages (~46% weight), fuel and light, housing, and miscellaneous items. Second, record prices in the base year and the current year. Third, assign weights — items that households spend more on get a higher weight, because they matter more to the cost of living. Fourth, apply a formula. The Weighted Average of Price Relatives formula is: add up (each item's price relative × its weight) and divide by the total weight. A price relative is simply (current price ÷ base price) × 100.

India uses three main indices that you must know for your board exam. The Consumer Price Index (CPI) tracks the prices paid by urban and rural households for their daily basket of goods — it is the headline inflation figure that the Reserve Bank of India (RBI) watches most closely. The Wholesale Price Index (WPI) tracks prices at the wholesale or producer stage, before goods reach shops — it often moves ahead of CPI, giving early warning of inflation. The Index of Industrial Production (IIP) is different: it does not measure prices but instead measures how much the industrial sector (manufacturing, mining, and electricity) is producing compared to the base year. A rising IIP signals a growing economy; a falling IIP signals a slowdown.

These indices are not just textbook numbers — they drive real policy. The RBI's official inflation target is 4%, with a tolerance band of ±2 percentage points (so 6% is the upper limit). When CPI crosses that band, the RBI raises interest rates to cool spending. Higher interest rates mean bigger EMIs on home loans and car loans — you can feel it in your household budget. When IIP falls sharply, the government may announce tax cuts or increased spending to revive production. Every budget headline and every RBI press release you read is, at its core, a story about index numbers.

Two construction methods you should be able to distinguish are the Laspeyres index (uses base-year quantities as weights) and the Paasche index (uses current-year quantities as weights). The Laspeyres method is simpler because base-year data is already collected; the Paasche method is more accurate because it reflects current consumption patterns, but requires fresh data every year. India's official indices mostly use a fixed-weight approach closer to Laspeyres, with the base year revised every few years — currently 2011–12 for WPI and 2012 for CPI. For your exam, practise computing both and be ready to explain why they may give slightly different answers for the same data.

An Indian example

In May 2022, India's retail inflation (CPI) surged to 7.04% — the highest in eight years — driven mainly by a sharp spike in food and fuel prices after global supply chains were disrupted by the Russia-Ukraine conflict. The RBI, whose upper tolerance limit is 6%, called an emergency Monetary Policy Committee meeting and raised the repo rate by 0.4 percentage points outside its usual schedule. Simultaneously, the government tracked WPI data, which hit 15.88% that same month — meaning wholesale prices were rising even faster than retail prices, warning that more CPI pressure was on the way. IIP data for that period showed industrial output growing strongly year-on-year (driven partly by a favourable base effect), so the economy was still expanding even as inflation ran hot. Policymakers used all three indices together: CPI told them consumers were hurting, WPI told them more pain was coming, and IIP reassured them the economy could handle a rate hike without crashing. By the end of 2022, after several rate hikes totalling 2.25 percentage points, CPI had cooled back below 6%. This episode shows exactly how index numbers are the instruments on the dashboard that policymakers use to steer the economy.

Key concepts covered

• Construction
• CPI, WPI, IIP
• Inflation

Common misconceptions to watch for

• Many students think an index of 150 means the item costs ₹150. It does not — an index of 150 means the price is 50% higher than it was in the base year. The actual rupee price depends entirely on what the base year price was; the index only tells you the relative change.
• Students often assume that if CPI rises, IIP must rise too, because 'the economy is doing well.' In reality these indices measure completely different things — CPI measures prices, IIP measures output — and they can easily move in opposite directions. When supply is disrupted (as during COVID or a bad monsoon), prices can spike (CPI up) while production falls (IIP down) at the same time.
• A common exam mistake is writing that 'the base year should always be the most recent year to stay current.' The whole point of a base year is that it stays fixed for several years so you can make meaningful comparisons across time. If the base year changed every year, a CPI of 108 in 2024 and a CPI of 108 in 2025 would mean completely different things and you could not compare them.