Hi and thanks for reaching out.

We take all trading pairs and group the into markets.


Ie all CNY/BTC from all exchanges.
We then use the standard weighted average calculation to get a volume-weighted average for this pair.
Here is the Wikipedia description and formula: http://en.m.wikipedia.org/wiki/Volume-weighted_average_price

Once we have that for each market, we collect the data together through the same methodology to get a global price.

Hope that helps.

We will be updating our methodology page to make this more clear.