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

A base year is the reference period (=100) against which an index is measured. In 2026 India shifted the Index of Core Industries, the Index of Industrial Production, WPI/PPI and the GDP series alike to base year 2022-23 [1][3][5][6] — a statistically necessary reset that nonetheless unsettles comparison across time.

Rationale for periodic revision

• Changing production structure: the new IIP series redrew sectoral weights from Gross Value Added shares of 2022-23, with manufacturing weights split using Annual Survey of Industries 2022-23 data [4]. An ageing base carries obsolete weights that distort measured growth.
• Capturing new activity: Iron Ore was added to the ICI basket, raising core industries from eight to nine [1].
• Methodological correction: the ICI now compiles steel from gross rather than net production, and retains only Raw Coal, excluding coal middling and washed coal to remove double counting [1].
• Expert scrutiny and coherence: revision followed a Technical Advisory Committee review [2], and simultaneous rebasing of ICI, IIP, WPI and GDP improves inter-index comparability [1][5][6].

Effect on inter-temporal comparability

• The ruler changes: ICI growth of 5.0% in June 2026 on the new series [1] cannot be set directly against old-series figures; every citation must name its series.
• Truncated history: the new ICI back series runs only from April 2023 [1], so claims about multi-year lows lack support.
• Estimated, not re-measured, history: older values are joined through linking factors, and MoSPI itself cautions against comparison across the base change [4].
• Mitigation: parallel running of both series, published back series, a public FAQ and the committee report [2][4] largely restore usable continuity.

Base-year revision is therefore a trade-off: short-run discontinuity purchased for long-run accuracy. The way forward lies in shortening revision cycles, publishing item-wise weights and methodology notes for every index, and extending back series — so that statistical modernisation strengthens, rather than interrupts, evidence-based policymaking.