1971   2.8
 1972   3.9
 1973   4.4
 1974   1.9
 1975   2.0
 1976   2.8
 1977   1.6
 1978   3.1
 1979   4.7
 1980   3.7
 1981   2.6
 1982   3.2
 1983   3.6
 1984   1.3
 1985   2.6
 1986   1.7
 1987   3.8
 1988   3.5
 1989   4.8
 1990   4.5
 1991   4.3
 1992   4.4
 1993   4.9
 1994   3.8
 1995   3.7
 1996   3.0
 1997   4.2
 1998   4.1
 1999   4.0
 2000   4.0