# Bilateral currency changes and effective exchange rates

Data Note · Monetary Economics
A Singapore-dollar movement against the U.S. dollar does not uniquely determine its movement against a currency basket. Other bilateral rates can move differently, and their weights determine the aggregate result. Treating one bilateral quotation as the effective exchange rate can therefore reverse the interpretation of external monetary conditions.

MAS describes its [policy framework](https://www.mas.gov.sg/monetary-policy/Singapores-Monetary-Policy-Framework) in terms of the Singapore dollar nominal effective exchange rate, or S$NEER, against a trade-weighted basket. A separate [Bank of England methodological explanation](https://www.bankofengland.co.uk/statistics/details/further-details-about-effective-exchange-rate-indices-data) describes the usual weighted-geometric construction of effective exchange-rate indexes. The hypothetical basket below demonstrates that arithmetic; it does not disclose or reconstruct the MAS policy basket.

## Keep the quotation direction consistent

Measure every bilateral rate as units of the partner currency obtained for one unit of the home currency. An increase then represents home-currency appreciation. Mixing this convention with home currency per partner unit would reverse some components and invalidate the aggregation.

Suppose a two-currency basket has fixed weights of 40 percent and 60 percent. The home currency depreciates by 2 percent against partner A but appreciates by 6 percent against partner B. Normalize both initial bilateral indexes and the basket to 100.

The basket relative is:

\[
0.98^{0.4}\times1.06^{0.6}\approx1.027245.
\]

The basket appreciates by about 2.72 percent despite the depreciation against A. An analyst observing only A's bilateral rate would describe a direction that does not match the stated basket.

The weights and bilateral changes are invented. They are not estimates of any Singapore policy weights or exchange-rate movement. Their purpose is to show why a liquid bilateral market can be important for transactions while remaining distinct from the currency basket used in policy analysis.

## The basket answers a particular exposure question

Trade weights aim to represent an aggregate economic relationship. A company's actual currency exposure may be very different. Its import invoices could be concentrated in one currency while its sales, funding and hedges are denominated in others. An appreciation of the aggregate index does not determine its net cash-flow effect.

Nor is the nominal effective rate a complete measure of price competitiveness. Relative domestic and foreign prices can change even when the nominal basket is stable. A real effective exchange-rate measure introduces an explicit relative-price adjustment and must identify the price indexes and weights used.

A change in the index reference period also needs interpretation. Rebasing an index from one reference year to another changes its displayed level without, in itself, changing the underlying bilateral movements. A level above 100 is not evidence that the currency is overvalued relative to an economic equilibrium.

## Policy implementation is not the same as the indicator

MAS states that its spot foreign-exchange intervention commonly uses the Singapore-dollar/U.S.-dollar pair. That implementation choice does not turn its trade-weighted intermediate target into a bilateral dollar target. The distinction between the traded instrument and the policy indicator is essential to interpreting operations.

For research on Singapore, the useful record names the effective-rate series, indicates whether it is nominal or real, identifies its reference period and retains its stated construction. A bilateral quotation can then be shown as additional information rather than substituted for the basket.

That approach also clarifies a business briefing. Discuss the aggregate exchange-rate environment using the relevant index, then examine the firm's own currency contracts and input exposures separately. A single dollar quotation cannot perform both tasks.

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American Economic Senate