The most important value of this index is the deep data history, which goes all the way back to July 2010. This deep historical data is based on data of the former exchange Mt. Gox and several other exchanges of that time.
The more recent data is based on data from 4 key cryptocurrency exchanges: Bitstamp, Coinbase, Gemini and Kraken. The exact formula for this recent data is (BITSTAMP:BTCUSD + COINBASE:BTCUSD + GEMINI:BTCUSD + KRAKEN:BTCUSD)/4.