Based on nominal exchange rates The index is computed as the
geometric mean of the bilateral exchange rates of the included currencies. The weight assigned to the value of each currency in the calculation is based on trade data, and is updated annually (the value of the index itself is updated much more frequently than the weightings). • w_{j,t} is the weight of currency j at time t • and \sum_{j=1}^{N(t)} w_{j,t} = 1
Based on real exchange rates The real exchange rate is a more informative measure of the dollar's worth since it accounts for countries whose currencies experience differing rates of inflation from that of the United States. This is compensated for by adjusting the exchange rates in the formula using the
consumer price index of the respective countries. In this more general case the index value is given by: :I_t = I_{t-1} \times \prod_{j = 1}^{N(t)} \left( \frac{e_{j,t} \cdot \frac{p_t}{p_{j,t}}}{e_{j,t-1}\cdot \frac{p_{t-1}}{p_{j,t-1}}} \right)^{w_{j,t}}. where • p_t and p_{t-1} are the values of the US consumer price index at times t and t-1 • and p_{j,t} and p_{j,t-1} are the values of the country j's consumer price index at times t and t-1 ==Federal Reserve Bank of St. Louis data==