Difference,Year
1.7,1998
1.8,1997
2.3,1996
2.4,1995
2.2,1994
2,1993
2.1,1992
2.2,1991
2.2,1990
2.4,1989
2.3,1988
2.2,1987
2.6,1986
2.2,1985
2.4,1984
2.6,1983
2.7,1982
2.5,1981
2.7,1980
2.3,1979
2.4,1978
2.4,1977
2.5,1976
2.4,1975