27. The Chapter Resources section of has data on Microsoft and the S&P 500 from 1986 to 2006.

a. Estimate Microsoft’s beta using linear regression over the periods 1987–1991, 1992–1996, 1997–2001, and 2002–2006.

b. Compare the four estimated betas. What do you conclude about how Microsoft’s exposure to systematic risk changed over that 20-year period? What do you think explains the change?