How google finance calculates beta of a stock - What is the proxy for the market? - What is the time period it uses for regression?
According to a comment on another post here:
". It regresses against the SP500 using MONTH-END closing prices for the last five years." - @Dimitri
There are some variants to calculate the beta of a stock.
If not fully documented at Google, in doubt you have to validate yourself. You will find a help to do this in the linked website. However, the results of the different calculation variants are usually quite similar.
Being a Google Finance user myself I was not able to figure out how it computes the beta. However, my best guess is that it's done in a way that is very similar to the methods used by Yahoo and Bloomberg. I.e. SP500 and 36 or 60 monthly observations.
In general, I would say that it does not really matter how the beta is computed since the betas on Google and Yahoo are only used as a quick scan. For a closer examination, it always better to create your own definition of beta. For example, if you want to figure out how risky apple is compared to other tech companies it makes more sense to regress against the NASDAQ than the SP500.
Yahoo: monthly 3 year observations (36 in total) against S&P 500 Bloomberg: monthly 5 year observations (60 in total) against S&P 500