60-Month Beta is the coefficient that measures the volatility of a stock's return relative to the S&P 500. A Beta of 1 means that the market and the stock historically move up or down together, at the same rate. That is, a 5% up or down move in the market should theoretically result in a 5% up or down move in the stock.
Beta is based on a 60-month historical regression of the return on the stock onto the return on the S&P 500:
Ri= a + ß(Rm) + e where Ri is the monthly total returns on the stock, a is the stock's Alpha, ß is the stock's Beta, Rm is the monthly total return of the market (S&P 500), and e is a random error term. A minimum of 12 monthly returns are required for this calculation.