Over the long term however, as with most statistics, the volatility might change and it therefore should be monitored on an ongoing basis. However, A University of Chicago study of thousands of stocks over 50 years now says it makes little , if any, difference . They did note that stocks with high book values relative to their stock market price did better than stocks with low book values relative to market price. Also, they confirmed that small companies tend to do better than large cap stocks. They suggest that current beta should be a reflection of past beta, firm size, debt ratio, variance of earnings, variance of cash flows, growth in earnings per share, market capitalization, dividend yield, debt to asset ratio and other ratios as well.
All betas are relative to one or another market proxy.
Naturally, something's beta will change if its is measured against different market indices. In summary, there probably should not be a wholesale reliance on beta as the only criteria used to determine the risk of a stock or portfolio. However, in the short time frame, and certainly for current review, it is a valuable number in comparing one major element of risk to that of other indices. Further, almost all rating services use either beta directly or something very similar- see Value Line, Morningstar, Lipper, etc. Every investor must know about beta.