Year	Number
1971	19.49805
1972	17.15719
1976	22.18701
1977	20.92774
1978	20.00152
1979	18.87419
1980	18.61517
1981	18.5293
1982	19.0916
1983	18.75617
1984	16.93677
1985	17.79819
1986	21.27483
1987	21.21038
1988	21.25672
1989	22.87327
1995	27.58942
1996	26.03491
1997	31.26114
1999	28.11663