All securities have weight 1? According to the website, the index is calculated by simply adding shareprices. That seems weird.
That's how I've put it together to start off with. There's definitely other methods and if you think one is best please please tell me. The one that hands the most utility to people is the ideal one. The only way to figure out which method is best is to ask people what's best.
I would definitely opt for a market-capitalization-weighted index. Right now, prices on BTCT range from well below a bit-cent (Labcoin, ActM) to several coins (AM). With price-weighting, even a doubling of, for example, the Labcoin price will move the index considerably less than 1%, while a 2% change in AM-PT price probably would achieve this.
1) Avg Top 25 Stock Price
2) 1% Model, Add together 1% market cap for each of the 25.
3) I like the S&P 500 Method (
http://www.investopedia.com/ask/answers/05/sp500calculation.asp).
4) Add them up. It's easy on the server!
2 and 3 are the same (other than that 2 limits to the top 25). They may give another value, but they'll always be proportional to eachother. 1 and 4 are the same as well, with the exception that you limit it to the top 25.
Essentially there are 3 easy ways to compute an index (and infinitely many more complex variations):
1. Add prices together. Super-simple, but a very poor representation of what is going on in the market, since stocks with a high base price completely outweigh stocks with a low base price, independent of the size of the company behind the stocks.
2. Add prices together, with prices of each stock normalized to their starting price. Basically set a starting time for the index and evaluate the prices relative to the price at the starting time. Advantage of this method over the previous one is that the base price of the stock doesn't matter, only its change since the starting time of the index.
3. Add market caps together (the S&P 500 method). This I consider the most informative index-method, as it weighs large companies more than small companies.
With each method, you can divide or multiply the final index by some constant to obtain a nicer number. This doesn't change the functionality of the method.