Don't be fooled by bogus data

Here are SP500\'s returns in 1957,1967,1977,1987,1997. I got the data from Yahoo finance.


12/31/1956 => 12/31/1957 46.67 => 39.99 Change= -14.3%
12/31/1966 => 12/31/1967 80.33 => 96.47 Change= 20.1%
12/31/1976 => 12/31/1977 107.46 => 95.1 Change= -11.5%
12/31/1986 => 12/31/1987 242.17 => 247.08 Change= 2.0%
12/31/1996 => 12/31/1997 740.74 => 970.43 Change= 31.0%