Year	Number
1971	25.11653
1972	24.20741
1973	23.59209
1974	21.64596
1975	20.82812
1976	20.04911
1977	19.46907
1978	19.3371
1979	18.8736
1981	18.72319
1984	15.6073
1986	16.76363
1987	16.59772
1988	16.63185
1989	16.75533
1991	16.59162
1992	16.56896
1993	16.58239
1995	18.14897
1996	18.072
1997	17.88299
1998	17.8757
1999	17.87578