You are right, but BTWTY have the purpose of being, as the first index I'll create, a very stable a robust one. Volume could be faked by an exchange to manipulated which coin comes in or out in the index just before the revision. That's why I'll only consider MC for this one. The revision is made once a month at the moment. So this kind of coin jumping up and down is very unlikely to end up in the index. We are actually defining a set of rules in the scripts to be sure no exotic moves of shit coins can affect the index.
All the explanations will be availbable on the web site. I entend on making it public as soon as I can send the price feed on the BTWTY test smartcoin on the testnet.
It shouldn't be long now, I'm almost there. Xeroc and hcf27 are also helping.