1950 1.06 
1955 1.18
1960  1.28
1961  1.31
1962  1.26
1963  1.31
1964  1.40
1965  1.40
1966  1.52
1967  1.54
1968  1.59
1969  1.60
1970  1.65
1971  1.77
1972  1.74
1973  1.84
1974  1.76
1975  1.76
1976  1.9
1977  1.87
1978  2.05
1979  2.00
1980  2.00
1981  2.04
1982 2.16
1983  2.1
1984  2.31
1985  2.32
1986  2.38
1987  2.36
1988  2.28
1989  2.43
1990  2.54
1991  2.46
1992  2.58
1993  2.48