Why Nigeria’s inflation numbers changed in December — and it’s not an error
By Victor Ejechi
Nigeria’s inflation figures for December 2025 were released on Thursday, and a closer look at the data reveals an unusual shift. The year-on-year inflation rates for earlier months in 2025, particularly November, no longer match the figures published just weeks earlier.
For instance, the National Bureau of Statistics had reported headline inflation of 14.45 percent for November 2025. However, in the December inflation report, the November figure was revised upward to 17.3 percent, alongside revisions to year-on-year inflation rates for other months from January to November 2025.
While this change may raise questions about consistency or data integrity, it reflects neither an error nor a problem with the data.
What happened is technical, but far less controversial than it appears. The shift reflects a methodological adjustment following the rebasing of Nigeria’s Consumer Price Index (CPI) to 2024 = 100. In simple terms, inflation was not recalculated because the numbers were wrong, but because the statistical base used to measure them was finalised.
A simple analogy helps explain this. Think of inflation like measuring height with a ruler. If you change where the ruler starts from, the measured height can change even if the person has not grown. That is essentially what happened to Nigeria’s inflation figures in December.
What changed
After rebasing the CPI, NBS faced a choice about how to anchor inflation comparisons. One option was to set December 2024 alone as the base month and compare December 2025 prices against it. That approach would have produced a sharp, misleading year-on-year jump in December 2025 inflation, driven largely by what economists call a base effect.
A base effect occurs when inflation appears unusually high or low because of what prices were doing a year earlier, rather than current price pressure. In this case, using a single month would have mechanically exaggerated inflation, even if underlying price movements were moderating.
A simple way to understand base effects is fuel prices. If petrol prices were unusually low in December last year, comparing today’s prices to that single month would make inflation look worse than it really is. The jump would come from the comparison point, not from a fresh surge in prices.
To avoid this distortion, NBS adopted a more stable approach known as index reference period maximisation. Instead of using one month, the bureau set the average CPI for all 12 months of 2024 equal to 100. This spreads the base evenly across the year and produces inflation rates that better reflect current economic realities.
Instead of judging inflation on a single unusual month, NBS used the average of all 12 months in 2024. This is similar to using a student’s average score over a full academic year, rather than judging performance on a single difficult exam.
“The December 2025 year-on-year Headline inflation rate, including all other sub-indexes were obtained through maximisation of the index reference period, that is, using a 12-month index reference period where the average CPI for the 12 months of 2024 is equated to 100,” NBS said.
“This is a departure from the single-month index reference period, in which December 2024 was set to 100, which would have produced an artificial spike in the December 2025 year-on-year inflation rate.
“This artificial spike is induced by the base effect, which is methodological, not structural, resulting in a rate that is not in tandem with current inflationary realities; hence the need to resort to the 12-month index reference period, by equating the entire 2024 to 100.
“This definitely affects the raising factor used for the re-referencing of the 2024 CPI series and the already released year-on-year inflation rates for January to November 2025.”
Why earlier 2025 inflation figures changed
Once the 12-month average for 2024 became the reference point, it affected how all 2024 CPI values were scaled. Because year-on-year inflation compares prices in 2025 with prices in 2024, the recalibration automatically changed inflation rates for January to November 2025.
This is why, for example, year-on-year inflation for November 2025 appears higher in the December report than in the November release. The earlier figures were calculated using a provisional reference structure, which was later replaced after the final rebasing methodology was applied.
Importantly, this does not mean inflation suddenly jumped or fell in real life. It means the measuring stick was refined.
Once the reference point changed, all earlier comparisons had to be recalculated. Keeping the old figures would have mixed two different measuring systems, which would have been statistically incorrect.
Is this internationally acceptable?
Yes. NBS explicitly stated that the adjustment follows international best practice, citing guidance from the International Monetary Fund’s CPI Manual and the ECOWAS Harmonised CPI framework. These manuals recommend index reference period maximisation after rebasing, especially in economies experiencing high inflation or volatile price movements.
“This process is in line with International Best Practice as contained in the Consumer Price Index Inter national Monetary Fund (IMF) Manual, specifically in Section 9.125 and the ECOWAS Harmonised CPI Manual, which address index reference period maximisation, following a rebasing exercise,” NBS said.
“The 2024 re-reference CPI and the revised year-on-year inflation rates for January to November 2025 can be found in the Excel Tables published together with this report on the NBS website.”
This means that statistical agencies in other countries have taken similar steps after rebasing price indices, often revising historical inflation rates to preserve consistency.
What this means for the public and analysts
The revised inflation figures do not signal data manipulation or reporting errors. They represent a shift from provisional to final measurements after a major statistical update.
For analysts, policymakers, and journalists, the implication is clear: the December 2025 inflation report supersedes earlier 2025 figures, and the revised series should be used for trend analysis, commentary, and forecasts going forward.
For households and businesses, the change does not alter lived experience. Food, transport, housing, and energy prices did not suddenly change because of a rebasing exercise. What changed is how inflation is measured and compared over time.
Victor Ejechi, Head of Insights SBM Intelligence
Related to this topic:
