Year	Number
1971	14.13043
1972	14.57906
1973	17.79885
1974	17.79048
1975	17.82101
1976	17.88851
1977	19.18537
1978	18.03279
1979	17.98319
1980	17.91045
1981	18.35656
1982	18.06761
1983	18.00537
1984	18.02727
1985	18.06835
1986	18.064
1987	17.98658
1988	18.08219
1989	17.94521
1990	17.91667
1991	18.01418
1992	18
1993	17.98849
1994	18
1995	18.01418
1997	18.06723