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The reference index value that applies at a time t is defined as the raw inflation index I(t) that applies at time t, but generally published after t with some index-specific publication delay Δt
In various applications, the value I(t) alone is not enough.
It must be accompanied by the so-called base value I(t₀) that equals the raw inflation index at some fixed time t₀ known as base time.
Typically, only the ratio I(t)/I(t₀) is relevant in calculating cash flow amounts that are contingent on the relative inflation index I(t)

It is also likely that the raw inflation index is published only for certain dates, which usually are on the first day of each month.
In that case, I(t) is still valid for any t by means of linear interpolation.

Furthermore, the published raw inflation index may be revised before being used as input to the calculation of the reference index.