Base year revision

Also called: Updating the base year · Topic: Inflation and Index Numbers: CPI, WPI, IIP and the Deflator · NCERT: Class 11, Ch 7 "Index Numbers"

Meaning

Base-year revision means moving an index's base year (or the national accounts' base year) to a newer year, from time to time. The basket (the fixed list of items tracked), the weights (how much each item counts) and the data sources are updated along with it, so the index matches what people buy and produce today.

It matters because an old base gives a wrong reading of inflation and growth. That wrong reading then flows into RBI interest-rate decisions, dearness allowance (DA) for workers, and real GDP.

• Index formula: Index in year t = (Value in year t ÷ Value in base year) × 100. The index equals 100 in the base year.
• Linking factor (used when the base changes): LF = Avg I_new ÷ Avg I_old, measured over an overlap year. Linked index = I_old × LF [2].

Explanation

Why a base must be revised

• Consumption patterns change.
• As incomes rise, the share of spending on food falls. This is Engel's law.
• Spending on services, mobile data and packaged foods grows.

• An old basket keeps giving food too much weight.

• Items disappear and new ones appear.

• In the new IIP (Index of Industrial Production), 64 item groups were dropped, such as kerosene, fluorescent tubes and CFLs, sewing machines and printing machinery [3].

• 120 new item groups were added, such as debit/credit cards, CCTV cameras, stents, vaccines, and aircraft and spacecraft parts [3].

• Better data sources become available. Examples are HCES 2023-24 (Household Consumption Expenditure Survey), ASI 2022-23 (Annual Survey of Industries), e-commerce prices, and CAPI (Computer Assisted Personal Interview, where prices are collected on tablets instead of paper).

• Fixed weights drift over time.
• A Laspeyres index keeps the base-year quantities (q₀) as weights: P_L = (Σp₁q₀ ÷ Σp₀q₀) × 100.
• When some goods become costlier, people switch to cheaper ones. A fixed-weight index does not see this switch.
• So Laspeyres tends to overstate inflation. Regular revision corrects this drift.

What makes a good new base year

• It should be a "normal" year. Avoid years with droughts, wars or booms.
• Worked example: rice costs ₹20/kg in a normal year and ₹40/kg in a drought year. Today it costs ₹30/kg.
• With the drought year as base: (30 ÷ 40) × 100 = 75. Prices seem to have fallen.

• With the normal year as base: (30 ÷ 20) × 100 = 150. Prices have really risen 50%.

• It should not be too far in the past. A 1960 basket compared with 2005 is not meaningful, because many 1960 items have vanished.

• Official test for IIP 2022-23: the base should be "a relatively stable economic period" and should match the base year of GDP and WPI [3].
• CPI 2024 separates three reference periods [2]:

Term	Meaning	CPI 2024 value
Index reference period	Year in which the index = 100	2024 = 100
Weight reference period	Period whose spending data give the weights	HCES 2023-24
Price reference period	Period in which base prices are collected	Calendar year 2024

Worked example: new weights change the inflation number

• CPI food weight (combined, old group structure) fell from 45.863 (CPI 2012) to 40.104 (CPI 2024) [2].
• Assume food prices rise 10% and all other prices rise 4%.
• Old weights: 0.4586×10 + 0.5414×4 = 4.59 + 2.17 = 6.75%

• New weights: 0.4010×10 + 0.5990×4 = 4.01 + 2.40 = 6.41%

• Prices changed by the same amount in both cases. CPI inflation is lower only because food now carries less weight.

Splicing: joining the old and new series

• Splicing joins the old series and the new series into one continuous series using a linking factor, so that comparisons over long periods are still possible.
• CPI 2024 method [2]:
• Both series are compiled for an overlap year, which is 2025.
• Each average is the geometric mean of the monthly indices in that year.
• Linking factors (general index): Rural 0.5222, Urban 0.5320, Combined 0.5267 [2].

• Worked example: an old-base (2012) CPI Combined reading of 190 × 0.5267 ≈ 100.1 on the 2024 base.

• IIP: LF = geometric mean of the old series in the new base year ÷ geometric mean of the new series in the new base year. It is released for each sector [3].

In India

• Who revises what (status as of Sep 2026):

Index	Compiler	Old (NCERT) base	New base
CPI-IW	Labour Bureau	2001	2016, released Oct 2020; LF 2.88 [7]
CPI Rural/Urban/Combined	NSO (MoSPI)	2012	2024 = 100, released 12 Feb 2026 [2][6]
WPI	Office of the Economic Adviser (OEA), DPIIT	2011-12	2022-23, approved 25 May 2026, released 15 Jun 2026 [4][5]
IIP	NSO (MoSPI)	2011-12	2022-23, planned release 1 Jun 2026 [3]
GDP	NSO (MoSPI)	2011-12	2022-23, new series scheduled for 27 Feb 2026 [6]
CPI-AL/RL	Labour Bureau	1986-87	2019 = 100 [8]

• CPI 2024 [2]:
• Weights come from HCES 2023-24.
• The number of weighted items rose from 299 to 358: goods from 259 to 308, and services from 40 to 50.
• Rural housing had no weight in CPI 2012. It now has 5.527 (group level, rural).
• It adds 12 online markets and follows the UN's COICOP 2018 classification: 12 Divisions, 43 Groups, 92 Classes and 162 Sub-classes.

• A back series for Jan 2013 – Dec 2024 was published on 12 Feb 2026.

• IIP 2022-23 [3]:

• Item groups rose from 407 to 463.
• Mining now covers 44 minerals (up from 29), and renewable electricity is shown separately.

• This is the 10th base revision: 1937 → 1946 → 1951 → 1956 → 1960 → 1970 → 1980-81 → 1993-94 → 2004-05 → 2011-12 → 2022-23.

• WPI 2022-23:

• Items rose from 697 to 957. Solar, wind and nuclear electricity were added [4][5].

• New Producer Price Indices (PPIs) were launched, including a Service PPI for 7 services [5].

• Future cycle: MoSPI plans to revise the CPI base every 3-5 years, depending on when HCES data become available [2].

Don't confuse with

• Base year vs base-year revision: the base year is the single year in which the index = 100. Base-year revision is the process of moving to a newer base, along with a new basket and new weights.
• Base-year revision vs splicing: revision builds a new series. Splicing uses a linking factor to join the old and new series into one continuous series.
• Index reference period vs weight reference period: in CPI 2024 the index = 100 in 2024, but the weights come from HCES 2023-24 [2]. They are not the same period.
• CPI-IW linking factor vs CPI 2024 linking factor: the CPI-IW factor 2.88 turns a new index into its old-base value (new × LF) [7]. The CPI 2024 factor 0.5267 turns an old index into its new-base value (old × LF) [2].

Prelims Hooks

• Linking factors: CPI-IW 2001 → 2016 = 2.88. So the first new-base index of 118 (September 2020) ≈ 340 on the old base [7]. CPI 2012 → 2024 (Combined) = 0.5267, calculated over the 2025 overlap year using geometric means [2].
• Weight sources: CPI 2024 uses HCES 2023-24. IIP 2022-23 sector weights use GVA at current prices, 2022-23 [2][3].
• Food weight in CPI 2024 (Combined) = 36.753 on the COICOP structure and 40.104 on the old structure, down from 45.863 in CPI 2012 [2]. Both figures are correct. The gap comes from classification, not from the data.
• Trap on compilers: WPI is compiled by the Office of the Economic Adviser, DPIIT, not MoSPI. CPI-IW and CPI-AL/RL are compiled by the Labour Bureau.
• Basket sizes: IIP 2022-23 has 463 item groups (120 added, 64 dropped). WPI 2022-23 has 957 items (up from 697) [3][4].
• CPI-AL/RL now uses base 2019 = 100, replacing 1986-87 [8].

Mains Points

• An old base misleads monetary policy.
• With food at about 46% weight, every vegetable price spike pushed headline CPI up sharply.
• This affected the RBI's flexible inflation targeting (a 4% target with a ±2% band).

• New weights (food about 36.75-40%) give a truer picture of household costs. The trade-off is that headline inflation may now react less to food shocks that still hurt poor households.

• Aligned bases give better real numbers.

• GDP, WPI and IIP are moving to 2022-23, and CPI to 2024, all during 2026.
• So deflators and real growth rest on the same economic structure.

• Earlier, old WPI weights used to deflate a service-heavy GDP could misstate real growth. A Service PPI is the long-term fix [3][5][6].

• Base revision is also a welfare issue (GS-II/GS-III).

• DA for central government employees and pensioners depends on CPI-IW and its linking factor.
• A wrong link or a biased basket directly changes real wages.
• A regular 3-5 year cycle linked to HCES rounds [2], together with CAPI, e-commerce prices and administrative data, answers earlier criticism of the credibility of India's statistics.

Related concepts

• Base year

Read more

• Issues in construction: base year, representativeness and base revision