| Year | Value | | 1991 | 3.9000000954 | | 1990 | 3.9000000954 | | 1989 | 3.9000000954 | | 1988 | 4.8000001907 | | 1987 | 4.8000001907 | | 1986 | 4.8000001907 | | 1985 | 4.8000001907 | | 1984 | 5 | | 1983 | 2.7999999523 | | 1982 | 2.7999999523 | | 1981 | 2.7999999523 | | 1980 | 1.8999999762 | | 1979 | 1.8999999762 | | 1978 | 1.7999999523 | | 1977 | 2 | | 1976 | 1.8999999762 | | 1975 | 1.8999999762 | | 1974 | 1.7999999523 |
|