The BDI - though a useful early-warning index - is highly volatile and exaggerates apparent ups and downs in trade.
BDI——虽然作为早期警报指数有用——总是具有不稳定性和夸大性，随交易其升降很大。
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The index is based on the idea of purchasing-power parity (PPP), which says currencies should trade at the rate that makes the price of goods the same in each country.
The firm started to sell $4.1 billion of “E-Mini” futures contracts based on the S&P 500 index through an algorithmic trade, taking account only of volume, not time or price.