1948    16.6
1949    16.2
1950    15.2
1951    14.2
1952    13.9
1953    12.5
1954    11.7
1955    11.5
1956    11.1
1957    10.3
1958     9.8
1959     9.3
1960     8.7
1961     8.1
1962     7.7
1963     7.1
1964     6.7
1965     6.4
1966     5.9
1967     5.5
1968     5.2
1969     5.1
1970     4.7
1971     4.5
1972     4.6
1973     4.5
1974     4.3
1975     4.1
1976     3.8
1977     2.8
1978     2.9
1979     2.8
1980     2.7
1981     2.5
1982     2.4
1983     2.5
1984     2.4
1985     2.2
1986     2.2
1987     2.1
1988     2.1
1989     2.0
1990     1.9
1991     1.9
