Base effect

Topic: Inflation and Index Numbers: CPI, WPI, IIP and the Deflator · NCERT: Beyond NCERT

Meaning

Base effect is the change in this year's year-on-year (y-o-y) inflation rate that comes only from the price level a year ago (the "base"), not from price changes happening now. A low base (prices were unusually low last year) makes this year's rate look high. A high base (prices spiked last year) makes this year's rate look low.

• Y-o-y inflation rate (%) = (Iₜ − Iₜ₋₁₂) / Iₜ₋₁₂ × 100. Here Iₜ is the index for this month and Iₜ₋₁₂ is the index for the same month last year. MoSPI uses this formula for CPI inflation (2026) [1].
• Change in y-o-y inflation ≈ Momentum now − Momentum a year ago (base effect) [3].

It matters because the headline inflation rate can rise or fall sharply even when today's price pressure has not changed. The RBI has to see through this before it decides the repo rate (the interest rate at which the RBI lends money to banks for a short time).

Explanation

How it works: the denominator problem

• The y-o-y rate compares two numbers: the price index today and the price index a year ago.
• The index a year ago is the denominator (the number we divide by). If it is unusually small or large, the rate changes, even when today's prices are the same.
• Low base → high inflation rate:
• Last year prices were unusually low (for example, a vegetable glut, meaning too much supply).
• The denominator Iₜ₋₁₂ is small.

• This year's rate looks large.

• High base → low inflation rate:

• Last year there was a price spike.
• The denominator is large.
• This year's rate looks small, even if prices are still rising now.

Momentum and base effect: the RBI split

• A 12-month window moves forward by one month each month. One new month comes in and one old month drops out.
• Momentum = the month-on-month (m-o-m) price change in the current month. This is the new month coming in [3].
• Base effect = the m-o-m price change in the same month a year ago. This is the old month dropping out of the 12-month window [3].
• The rule:
• If momentum now is bigger than momentum a year ago, y-o-y inflation rises.

• If momentum now is smaller than momentum a year ago, y-o-y inflation falls.

• Favourable base effect: a big price rise a year ago drops out, so headline inflation falls.

• Unfavourable (adverse) base effect: a small rise or a fall a year ago drops out, so headline inflation rises.

Worked example: same price today, very different rates

• Index values: June 2024 = 100. July 2024 = 104, a one-month spike of +4%. June 2025 = 105. July 2025 = 105.5.
• June 2025 inflation = (105 − 100)/100 × 100 = 5%.
• July 2025 inflation = (105.5 − 104)/104 × 100 ≈ 1.44%.
• Split:
• Momentum in July 2025 = (105.5 − 105)/105 ≈ +0.48%.
• Base effect = the +4% of July 2024 dropping out.

• 0.48 − 4 ≈ −3.5 points. So inflation fell from 5% to about 1.4%.

• The lesson: prices still rose in July 2025, but the headline rate fell sharply. This is a favourable base effect.

• Low-base version: if July 2024 had fallen to 98 instead, July 2025 inflation = (105.5 − 98)/98 × 100 ≈ 7.65%. The same price today gives a much higher rate.

What makes base effects large

• Volatile items in the basket. Food prices swing a lot with the monsoon, heatwaves and unseasonal rain.
• Perishable goods. TOP (tomato, onion, potato) spoil quickly and India has thin storage for them. A small supply loss causes a big price jump, and that jump becomes next year's high base.
• Weight of those items. The bigger the share of volatile items in the index, the bigger the base effects in headline CPI.

In India

• Who measures it:
• MoSPI (Ministry of Statistics and Programme Implementation) publishes the monthly CPI (Consumer Price Index: retail prices that households pay) and computes y-o-y inflation with the formula above [1].

• WPI (Wholesale Price Index: prices in bulk trade) is published monthly by the Office of the Economic Adviser, DPIIT. Base effects affect WPI inflation in the same way.

• New CPI series and the base:

• India's new CPI has base year 2024 = 100. It was released on 12 February 2026, starting with January 2026 data [1][2].

• To calculate y-o-y inflation you need last year's index. So the old series (2012) is joined to the new one using a linking factor. The general-level linking factor (Combined) is 0.5267, and a linked back series from January 2013 is available [1].

• How the RBI uses it:

• May 2021: the RBI said high momentum across food, fuel and core items, together with an unfavourable base effect, pushed headline inflation up [3].

• September 2024: inflation rose again after two months below target. The RBI said an adverse statistical base effect was made worse by a new rise in food price momentum [4].

• Why the RBI cares: under Section 45ZA of the RBI Act, the Central Government, in consultation with the RBI, sets a CPI inflation target every five years [7]. The target notified on 5 August 2016 was 4%, with a band of 2%–6% [7]. A base effect alone can push CPI across these limits, so the RBI has to tell a temporary jump apart from real price pressure.

• Food weight matters: Food and Beverages weight (Combined) fell from 45.86% (CPI 2012) to 36.75% (CPI 2024, COICOP structure) [1]. Food is the most volatile part of CPI, so a smaller food weight should reduce how much food base effects move headline CPI.

Don't confuse with

• Base year: the reference year when the index is set to 100 (2024 = 100 for the new CPI) [1]. Base effect is about the index level 12 months ago, not the base year of the series.
• Momentum: the m-o-m price change this month. It shows the real price pressure now. Base effect is the m-o-m change a year ago that is dropping out.
• Disinflation: inflation slows but prices still rise (6% → 4%). A favourable base effect can cause disinflation in the numbers, but it only describes the rate. It does not explain why the rate fell.
• Deflation: the price level itself falls and inflation is negative. A high base can make y-o-y inflation very low even while prices keep rising. That is not deflation.

Prelims Hooks

• High base → lower y-o-y rate; low base → higher y-o-y rate, even with the same price level today.
• Y-o-y CPI inflation (MoSPI): (Iₜ − Iₜ₋₁₂)/Iₜ₋₁₂ × 100 [1]. Comparing the same month in both years also removes most seasonal effects.
• RBI's split: momentum = m-o-m change this month. Base effect = m-o-m change in the same month last year [3]. Change in y-o-y inflation ≈ momentum now − momentum a year ago.
• Trap: a fall in headline inflation because of a favourable base effect does not mean prices fell. Momentum can still be positive.
• Trap: "base effect" and "base year" are different things. The new CPI base year is 2024 = 100, released 12 February 2026 [1][2].
• Inflation target: 4% CPI with a band of 2%–6%, set by the Central Government in consultation with the RBI under Section 45ZA, RBI Act [7].

Mains Points

• Reading headline inflation carefully: a sharp fall in y-o-y CPI can come mostly from a favourable base effect while momentum stays positive.
• If the RBI cuts rates on that fall, it may cut too early.
• If it ignores an adverse base effect that comes with rising momentum (as in 2021 [3] and September 2024 [4]), it may act too late.

• The momentum and base-effect split protects the credibility of the 4% (±2%) target [7].

• Supply shocks and base effects: food spikes caused by the monsoon, heatwaves or TOP perishability create large base effects a year later. The repo rate cannot fix these supply shocks.

• The lower food weight in CPI 2024, 36.75% compared with 45.86% in CPI 2012 [1], makes headline CPI a better guide to demand pressure.

• Supply-side tools are still needed to smooth these swings, such as Operation Greens (announced in Union Budget 2018-19 with ₹500 crore) [5] and MIEWS price alerts [6].

• Communication and public understanding: a low headline rate from a high base does not mean relief for households, because prices are still rising. Governments and the RBI should report momentum along with headline figures. This affects how people read DA revisions, wage bargaining and claims that inflation has been "controlled".

Related concepts

• Inflation
• Inflation rate
• Food inflation
• Creeping inflation
• Galloping inflation
• Hyperinflation
• Disinflation
• Deflation
• Reflation
• Skewflation

Read more

• Measuring inflation: rate, base effect and the vocabulary of price change