What it means
Index numbers exist because comparing raw values across many years is awkward. Setting one period as the base and expressing everything relative to it turns a long series into something readable, so an index of 115 immediately says that the measure is 15% above where it stood in the base period.
Base periods are also used to strip inflation out of financial figures. Dividing a nominal amount by the price index for the same year, then multiplying by 100, restates it in base-period money and allows a genuine comparison of purchasing power across time.
Choosing the base matters more than most people assume. It should be a reasonably normal period, since basing an index on a boom or a crisis year builds a distortion into every subsequent figure, and statistical agencies periodically rebase their indices to keep the reference recent and the weightings current.
Rebasing changes the index numbers but not the underlying reality, which regularly confuses readers who see a familiar series suddenly restart near 100. When comparing two indices, the first question should always be whether they share the same base period, because two series on different bases cannot be read side by side without conversion.
In practice
Real-world examples.
Example
A national statistics agency rebases its consumer price index from 2015 to 2020, resetting the 2020 value to 100. Reported inflation rates are unchanged, but every historical index number is restated, and analysts must convert older figures before comparing them.
Example
A retail chain sets the year before a major store refurbishment programme as the base period for its sales-per-square-metre index. Each subsequent year is expressed against that base so the board can see the cumulative effect of the programme rather than year-by-year noise.
Example
A trade union negotiates a wage agreement linked to a published cost of living index with an explicit base period written into the contract. Both sides agree in advance what happens if the agency rebases the index mid-agreement.
Formula
Calculation
Index = (current period value / base period value) x 100. Real value in base-period money = nominal value / (index / 100).
A company sets year 1 as the base period for its distribution cost index, with total distribution cost that year of $250,000. In year 4 the same activity costs $287,500.
Index for year 4: ($287,500 / $250,000) x 100 = 1.15 x 100 = 115.
That tells management distribution costs are 15% above the base period. To check whether that is a real increase, the finance team deflates it by a freight price index that stands at 118 in year 4 on the same base.
Real cost in base-period money: $287,500 / (118 / 100) = $287,500 / 1.18 = $243,644.
In real terms distribution costs are slightly below the base year, so the apparent 15% rise is entirely explained by freight prices and not by any deterioration in the company's own efficiency.Case study
Seen in the real world.
Thornbury Foods is an illustrative, fictional food producer that built an internal cost index to track whether its factories were becoming more efficient. It chose the previous financial year as the base period without much discussion, and that year happened to include a three-month energy contract at unusually low fixed prices.
Every subsequent quarter therefore showed costs rising against an artificially low base, and the operations team spent two years defending numbers that made them look progressively worse. When a new financial controller reviewed the index, she rebased it to a normal year and produced a like-for-like series, which showed that unit costs excluding energy had actually fallen 6% over the period.
The illustrative lesson is not that the original index was wrong, since the arithmetic was correct throughout, but that the base period had been chosen carelessly. A reference period should be typical, clearly documented and reviewed whenever the business or its cost structure changes materially.
Watch out
Common mistakes.
- Choosing an unusual year as the base. A boom, a crisis or a year with a one-off contract will distort every comparison that follows.
- Comparing two indices with different base periods. The numbers look comparable but are not, and one series must be converted before any conclusion is drawn.
- Reading a rebased index as a change in the underlying data. Rebasing only changes the reference point, not the growth rates or the real values behind it.
Questions
People also ask.
What does an index value of 100 mean?
It means the measure is exactly at its base period level, so 112 is 12% above the base and 94 is 6% below it.
How often should a base period be updated?
Statistical agencies typically rebase every five years or so, and internal indices should be reviewed whenever the business changes enough that the old base is no longer representative.
Can I convert an index from one base to another?
Yes, divide each value by the value of the new base year and multiply by 100, which preserves all the growth rates in the series.
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