Examine the rationale behind periodic base-year revisions of key economic indices (IIP, CPI, ICI) in India. How do such revisions affect inter-temporal comparability of data?

A base year fixes the reference structure — weights, item basket and the index value of 100 — against which later output or prices are measured. MoSPI's ongoing shift of GDP, IIP and CPI, alongside DPIIT's revision of the Index of Core Industries, marks India's widest statistical realignment in over a decade.

Rationale for periodic revision

• Changed consumption pattern: the CPI basket, rebased from 2012=100 to 2024=100, draws its weights from HCES 2023-24, raising weighted items from 299 to 358 [3].
• Changed production structure: the IIP (2022-23) basket covers 1,042 products in 463 item groups against 839 in 407 earlier, adding 120 new item groups [2].
• New activities: ICI added Iron Ore as a ninth core industry, with Electricity displacing Refinery Products as the largest-weight sector [4].
• Methodological correction: ICI now uses gross steel production for consistency with IIP, and retains only raw coal to remove double counting [4].
• Stated official aim — enhancing relevance, accuracy and international comparability — plus a common 2022-23 anchor that lets GDP, IIP, WPI and ICI be read coherently [1].

Impact on inter-temporal comparability

• Index levels across bases are not directly comparable; revised weights alter growth arithmetic even for identical physical output.
• Back series are truncated: the new ICI extends only to April 2023 [4], limiting long-run trend analysis.
• Provisional-to-final revision adds short-run noise — June 2026 ICI growth moved from a provisional 5.0% [4] to a final 6.0% [5].

Mitigating safeguards

• Linking factors, computed over the overlapping year 2025 when both CPI series co-exist, splice the two [3].
• Back-cast series (CPI: January 2013–December 2024) restore continuity [3], while Technical Advisory Committees vet methodology [2].

Base-year revision is therefore not a break with the past but a re-anchoring of measurement to present-day reality. The way forward lies in institutionalising a fixed revision cycle with simultaneous release of long back series, so that statistical modernisation strengthens rather than strains evidence-based policymaking.