Currency commentary usually quotes one exchange rate against one other currency. That figure can move sharply while the currency's broad position is barely changing.
Why a single bilateral rate misleads
A rate between two currencies moves when either side changes. A currency can appear to weaken simply because the other currency has strengthened against everything.
For a company that sells into a dozen countries, one bilateral rate describes only a slice of its revenue. The rest may be moving in the opposite direction entirely.
A trade-weighted index solves this by averaging a currency against a basket of others. The result describes the currency's position against the world rather than against one counterpart.
How the weights are constructed
Each partner currency enters the basket in proportion to how much trade the country conducts with that partner. A neighbour absorbing a large share of exports carries far more weight than a distant small market.
Some versions adjust further for third-market competition, recognising that two exporters may compete in a country neither of them borders. The weight then reflects rivalry as well as direct flows.
Weights are revised periodically as trade patterns shift. An index built on trade shares from a decade ago would misstate the exposure of most economies today.
What movement in the index implies for firms
A rising trade-weighted index means domestically produced goods are becoming more expensive for foreign buyers across the board. Exporters face pressure on either volumes or margins.
A falling index does the reverse, raising the home-currency value of foreign earnings while making imported inputs dearer. The net effect depends on how much of a firm's cost base is imported.
Sectors respond at different speeds because contracts roll at different intervals. Commodity producers reprice almost immediately, while engineering firms working to multi-year contracts feel the change much later.
Why real and nominal versions differ
The nominal index tracks exchange rates alone. The real version adjusts for differences in price inflation between the country and its partners.
Inflation running faster at home erodes competitiveness even when the exchange rate is unchanged, because domestic costs are rising in absolute terms. The real index captures that, the nominal one does not.
Policymakers watch the real measure because it approximates actual competitiveness. Markets often react to the nominal one because it updates continuously.
The limits of the measure
An average conceals distribution. A firm concentrated in one export market cares about that bilateral rate, whatever the basket is doing.
The index also assumes trade in finished goods, whereas much modern trade involves components crossing borders repeatedly. Definitions and methods vary between the institutions that publish these series.
Two published indices for the same currency can move by different amounts on the same day. Comparing levels across providers is meaningless, and only the direction and rough scale are reliable.