Base year

Also called: Base period · Topic: Inflation and Index Numbers: CPI, WPI, IIP and the Deflator · NCERT: Class 11, Ch 7 "Index Numbers"; Class 12, Ch 2 "National Income Accounting"

Meaning

The base year (also called the base period) is the reference year that an index number or a real (constant-price) figure is compared against. An index is set to 100 in the base year. For national accounts such as real GDP, the base year's prices are used to value output in every other year.

• Formula: Index in year t = (Value in year t ÷ Value in base year) × 100
• Why it matters: the base year sets the basket of items, their weights and the starting prices. If the base year is abnormal or outdated, the inflation and growth numbers will be wrong. Those wrong numbers then feed into RBI interest-rate decisions, dearness allowance (DA) for workers, and real GDP.

Explanation

How the base year works

• Index number: a single number that shows how a group of related things (prices or production) changes over time, measured against a reference period.
• The base year is not only about prices:
• Price index (CPI, WPI): compares prices with base-year prices.
• Volume index (IIP, Index of Industrial Production): compares physical output with base-year output. The IIP officially measures "changes in the volume of production" of an item basket, compared with its base year [3].

• Real GDP: values each year's output at base-year (constant) prices, so that price rises are taken out.

• In a Laspeyres index, the base year also supplies the weights:

• P_L = (Σp₁q₀ ÷ Σp₀q₀) × 100, where q₀ = base-year quantities.
• The Paasche index uses current-year quantities (q₁) instead: P_P = (Σp₁q₁ ÷ Σp₀q₁) × 100.
• The two differ only in their weights.

What makes a good base year (NCERT rules)

• Rule 1: it should be a "normal" year. Avoid droughts, wars and booms.

• Worked example:

  • Rice costs ₹20/kg in a normal year and ₹40/kg in a drought year. Today it costs ₹30/kg.
  • Drought year as base: (30 ÷ 40) × 100 = 75. Prices seem to have fallen.
  • Normal year as base: (30 ÷ 20) × 100 = 150. Prices have actually risen 50%.
  • A bad base year makes inflation look like deflation (falling prices).

• Rule 2: it should not be too distant.

• Comparing 1993 with 2005 is meaningful. Comparing 1960 with 2005 is not, because many items in the 1960 basket no longer exist.

• Proof from the new IIP: 64 item groups were dropped (kerosene, fluorescent tubes and CFLs, sewing machines, printing machinery), and 120 were added (debit/credit cards, CCTV cameras, stents, vaccines, aircraft and spacecraft parts) [3].

• 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].

Base-year revision and why an old base drifts

• Base-year revision: moving an index (or the national accounts) to a newer base year, so that the basket, weights and data sources match what people buy and produce today.
• Why a fixed base drifts:
• Laspeyres keeps the old weights.
• When some goods get relatively costlier, people switch to cheaper ones, but the index does not see this.

• So an old-base Laspeyres index tends to overstate inflation. Paasche tends to understate it.

• Why consumption changes:

• As incomes rise, the share of spending on food falls (Engel's law).

• Spending on services, mobile data and packaged foods grows.

• Worked example: new weights change the inflation number.

• Assume food prices rise 10% and all other prices rise 4%.
• Old CPI 2012 weights (food 45.86%): 0.4586×10 + 0.5414×4 = 6.75%
• New CPI 2024 weights (food 40.10%, old group structure): 0.4010×10 + 0.5990×4 = 6.41% [2]
• The price changes are identical, but the new base gives lower inflation because food now carries less weight.

Splicing: linking the old base to the new base

• Splicing: joining the old series and the new series into one continuous series using a linking factor (LF), so that long-run comparisons are still possible.
• CPI 2024 method: both series are compiled for an overlap year (2025). LF = Avg I_new ÷ Avg I_old, where each average is the geometric mean of that year's monthly indices [2].
• Linking factors (general index): Rural 0.5222, Urban 0.5320, Combined 0.5267 [2].
• Example: an old-base (2012) CPI Combined reading of 190 becomes 190 × 0.5267 ≈ 100.1 on the 2024 base.

In India

• Who compiles what:
• NSO, MoSPI: CPI (Rural/Urban/Combined), IIP and GDP.
• Labour Bureau: CPI-IW (industrial workers) and CPI-AL/RL (agricultural and rural labourers).

• Office of the Economic Adviser (OEA), DPIIT, Ministry of Commerce and Industry: WPI.

• Current base years (status as of Sep 2026):

Index	NCERT base	Revised base
CPI-IW	2001	2016, released Oct 2020 (September 2020 index); LF 2.88 [7]
CPI Rural/Urban/Combined	2012	2024 = 100, released 12 Feb 2026 [2][6]
WPI	2011-12	2022-23, approved 25 May 2026, released 15 Jun 2026 [4][5]
IIP	2011-12	2022-23, planned release 1 Jun 2026; 10th base revision [3]
GDP	2011-12	2022-23, new series scheduled for 27 Feb 2026 [6]
CPI-AL/RL	1986-87	2019 = 100 [8]

• CPI 2024 separates three "reference periods" [2]:
• Index reference period (index = 100): 2024
• Weight reference period (source of the spending weights): HCES 2023-24 (Household Consumption Expenditure Survey)

• Price reference period (when base prices were collected): calendar year 2024

• What the new base changed:

• CPI food and beverages weight: 45.863 (2012) → 40.104 (2024) on the old group structure, and 36.753 on the new COICOP structure (the UN's system for classifying consumer spending) [2].
• CPI weighted items: 299 → 358 [2].
• IIP item groups: 407 → 463 [3]. IIP sector weights come from GVA (Gross Value Added) at current prices in 2022-23 [3].

• WPI items: 697 → 957 [4].

• History of IIP bases: 1937 → 1946 → 1951 → 1956 → 1960 → 1970 → 1980-81 → 1993-94 → 2004-05 → 2011-12 → 2022-23 [3].

• CPI history: MoSPI's CPI started in January 2011 with base 2010 = 100, then moved to 2012, and now to 2024 [2].
• Future cycle: MoSPI plans to revise the CPI base every 3-5 years, in line with global practice and depending on when HCES data become available [2].

Don't confuse with

• Weight reference period / price reference period: the base year (index reference period) is where the index = 100. The weights may come from a different period: for CPI 2024, the weights come from HCES 2023-24 while the index base is 2024 [2].
• Linking factor (splicing): the base year is the new anchor. The linking factor is only the bridge that converts figures between the old and new series. Check which way it works: CPI-IW's 2.88 converts new → old-base (new × LF) [7], while CPI 2024's 0.5267 converts old → new-base (old × LF) [2].
• Current year / current prices: nominal GDP values output at each year's own prices. Real GDP values it at base-year (constant) prices. Only real GDP removes the effect of price rises.
• Laspeyres vs Paasche: both compare with the base year's prices. Only Laspeyres also uses base-year quantities as weights; Paasche uses current-year quantities.

Prelims Hooks

• The index value in the base year is always 100. Index in year t = (Value in year t ÷ Value in base year) × 100.
• Current bases: CPI (R/U/C) 2024 = 100; CPI-IW 2016; CPI-AL/RL 2019; WPI, IIP and GDP 2022-23 [2][3][4][6][7][8].
• Trap: WPI is compiled by the OEA, DPIIT, not MoSPI. CPI-IW and CPI-AL/RL are compiled by the Labour Bureau [4][7][8].
• The CPI 2012 → 2024 linking factor (Combined) is 0.5267, calculated on the 2025 overlap year using geometric means. The CPI-IW 2001 → 2016 linking factor is 2.88 [2][7].
• The move of IIP to 2022-23 is its 10th base revision. The official test is a "relatively stable economic period" that matches the GDP and WPI base [3].
• Food weight in CPI 2024 (combined, COICOP structure) is 36.75%. On the old structure it is 40.10%, down from 45.86% in CPI 2012 [2].

Mains Points

• An old base year distorts policy.
• The CPI 2012 basket gave food about 46% weight.
• So every vegetable price spike pushed headline inflation up sharply, which in turn shaped RBI's flexible inflation targeting (4% ± 2%).

• The 2024 base (food about 36.75-40%, more services) reflects household costs more accurately. The trade-off: the index now reacts less to food shocks that still hurt poor households the most [2].

• Aligned base years give better real numbers.

• GDP, WPI and IIP are all moving to 2022-23 (and CPI to 2024), so deflators and real growth now rest on the same economic structure.
• Earlier, old WPI weights used to deflate a service-heavy GDP could misstate real growth.

• The new Service PPI (Producer Price Index for services) is the long-term fix [3][5][6].

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

• DA for central employees and pensioners, and wage indexation, depend on CPI-IW and its linking factor [7].
• A wrong link or an outdated basket directly changes real wages.
• A regular 3-5 year cycle tied to HCES rounds [2], along with CAPI (tablet-based price collection), e-commerce prices and administrative data, strengthens the credibility of India's statistics.

Related concepts

• Base year revision

Read more

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