What is Beta Equation Security?

The market beta of a security is determined as follows: Regress excess returns of stock y on excess returns of the market. The slope coefficient is beta. Define n as number of observation numbers. Beta= [(n) (sum of [xy]) ]-[ (sum of x) (sum of y)]/ [(n) (sum of [xx]) ]-[ (sum of x) (sum of x)] where: n = # of observations (usually 36 to 60 months)