INSEE decision
Statistical institutes regularly change the reference period of the price indices they publish. By convention, the value of the index for the year of “rebasing” is set at 100.
INSEE has rebased the Consumer Price Index (CPI) as from February 2026: the reference period of the CPI indices has moved from 2015 to 2025.
On 18 February 2026, the institute published the final indices for January 2026, in particular the French CPI excluding tobacco, which is used in the calculation of the indexation coefficients of OATi.
Impact of the change in the CPI reference year on bonds indexed to the Consumer Price Index
The change in the reference year of the CPI excluding tobacco does not affect the indexation coefficient of OATi.
All cash flows of these bonds are multiplied by an indexation coefficient equal to the ratio between the daily inflation reference on the payment date of the cash flow and the daily inflation reference on the bond’s issue date.
The daily inflation reference is itself calculated by linear interpolation of the CPI excluding tobacco. Both references must be comparable even when a change of base occurred between the two dates.
The linking will be carried out using the CPI for December 2025, which corresponds to the daily inflation reference of 1 March 2026. Consequently, coefficients calculated using previous bases (up to the coefficients of 1 March 2026) remain unchanged. Price references on base 2025 are used from the coefficients of 2 March 2026 onwards.
The conversion factor between the daily inflation references on “base 15” and “base 25” is as follows:
Consequently
The indexation mechanism of an OATi is not affected by a rebasing, as it ensures a seamless transition between the two bases.
Information:
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