from:
http://us.ishares.com/misc/faq.htm
...and more on different windowing methods, here.
http://answers.yahoo.com/question/index?qid=20070518164421AA4g1qM
"Q. How is beta calculated?
A. Beta is the covariance of a stock or fund in relation to the rest of the stock market. To calculate Beta, divide the covariance [the correlation between two variables multiplied by the standard deviation of the two variables] of the index and the S&P 500 Index by the variance [the measure of the dispersion of a distribution] of the S&P 500 (covariance (index returns, S&P 500 Index returns)/variance(S&P 500 Index returns)). This calculation uses monthly returns data from the previous 10 years or since inception, whichever comes first. (hide answer)
Q. Why is the beta displayed on iShares.com different from other sources?
A. When it's available, iShares calculates a beta based on one year of daily returns history for each fund. This daily beta can result in a value different from a monthly beta, especially if the two values use different lengths of history. Other sources commonly use three or five years of monthly returns for the beta calculation. (hide answer)
Q. Why is beta not displayed for some funds on iShares.com?
A. Since all betas on iShares.com are calculated relative to the S&P 500 Index, which represents the US equity market, it would be inappropriate for calculating fixed income and precious metal betas. So, beta isn't listed for those fixed income funds and the trusts. (hide answer)"
