What it means
An index compresses a lot of data into one figure. Rather than following the prices of 500 companies or 700 grocery items individually, you follow a single series that summarises them, and the movement of that series tells you what the whole group did.
The base value is arbitrary but essential. Setting the starting period to 100 means every subsequent reading is instantly readable as a percentage change from the base, which is why an index level of 143.2 tells you at once that the basket is 43.2% above its base period.
Weighting is where indices get interesting. A simple index treats every item equally; a weighted index gives bigger items more influence, which is why a market-capitalisation weighted stock index moves more when a very large company falls 2% than when a very small one falls 20%.
Businesses use indices in more places than most people notice. Commercial leases are often uprated by a published inflation index, long-term supply contracts include index-linked price adjustment clauses, and executive bonus schemes frequently measure share performance against a sector index rather than in absolute terms.
The main nuance is that an index measures relative change, not absolute level. Knowing that a construction cost index sits at 128 tells you costs are 28% above the base period, but tells you nothing at all about what a square metre of building actually costs today.
Indices are also periodically rebased and their contents revised. When the composition of a basket changes, the index provider adjusts the divisor so the series does not jump artificially, which is why comparing two very old index readings across a rebasing needs care.
In practice
Real-world examples.
Example
A commercial landlord and tenant agree a five-year lease with rent uprated annually by a published consumer price index. When the index moves from 112.5 to 116.4, the rent rises by 3.47%, so an $80,000 annual rent becomes $82,773 for the following year.
Example
A construction contractor prices a two-year infrastructure job with a materials index adjustment clause. When the steel component of the index rises 14% in the first year, the contract allows an automatic price uplift rather than forcing the contractor to absorb a cost increase nobody could have forecast at tender.
Example
A listed retailer's board sets executive share awards to vest only if total shareholder return beats a retail sector index over three years. The share price rises 22% but the sector index rises 31%, so the awards lapse despite what looked like a strong absolute result.
Formula
Calculation
The general index formula is:
Index value = (current period value / base period value) x 100
Take a small market-capitalisation weighted index of three companies. In the base period their market values are:
Company A: $600,000,000
Company B: $300,000,000
Company C: $100,000,000
Total base value: $1,000,000,000, set to an index of 100
Twelve months later the market values are:
Company A: $660,000,000
Company B: $315,000,000
Company C: $95,000,000
Total current value: $660,000,000 + $315,000,000 + $95,000,000 = $1,070,000,000
Index value = ($1,070,000,000 / $1,000,000,000) x 100 = 107.0
The index has risen 7.0% over the year. Notice that Company C fell 5%, from $100,000,000 to $95,000,000, yet the index still rose, because Company A's $60,000,000 gain outweighed Company C's $5,000,000 loss. That is weighting doing its work: the largest constituent drives the result.Case study
Seen in the real world.
This is an illustrative, invented example. Northgate Facilities Group, a fictional cleaning and maintenance contractor, priced multi-year contracts with public bodies at fixed rates for the whole term. In its fourth year, wage inflation ran well ahead of anything management had modelled and three of the five largest contracts turned loss-making, contributing an operating loss of $840,000 on $19,000,000 of revenue.
The finance director rebuilt the pricing model around an index. New contracts carried a clause tying 70% of the contract value, the labour element, to a published wage index, with an annual adjustment applied each April. If the index rose 4.2%, the contract price rose by 70% x 4.2% = 2.94%.
Two years later the same wage pressure arrived again, but the effect on margin was minor because most of the increase passed through automatically. The illustrative point is that an index is not just a number on a financial news page; it is a practical instrument for shifting a risk neither party can control onto a shared, published measure.
Watch out
Common mistakes.
- Reading an index level as a price. An index of 128 is a relative measure against its base period, not a dollar figure for anything.
- Comparing index readings across a rebasing without adjusting. Providers reset the base value periodically, and a naive comparison of an old series with a new one can produce a nonsense answer.
- Assuming an index represents everything in its market. Most indices track a selected sample, so a share index of the largest 100 companies says little about small businesses in the same economy.
Questions
People also ask.
What does an index of 100 mean?
It marks the base period from which change is measured, so 100 is simply the starting line rather than any indication of value being high or low.
Why do weighted and unweighted indices give different answers?
A weighted index lets large constituents dominate the result, while an unweighted index gives a small item the same influence as a huge one, so the two can move in opposite directions in the same period.
Can we build our own internal index?
Yes, and many businesses do, tracking things like cost per delivery or revenue per store against a base year of 100 so that trends are readable without wading through raw numbers each month.
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