Citing the Yahoo Finance HelpYahoo Finance Help page,
[Beta: The Beta used is Beta of Equity. Beta is the monthly price change of a particular company relative to the monthly price change of the S&P500. The time period for Beta is 3 years (36 months) when available.][1]
Beta: The Beta used is Beta of Equity. Beta is the monthly price change of a particular company relative to the monthly price change of the S&P500. The time period for Beta is 3 years (36 months) when available.
Regarding customised time periods, I do not think so. [1]: http://tinyurl.com/c9thnab
