Year	Number
1971	12.35647
1972	13.5235
1973	13.23941
1974	14.06831
1975	13.49206
1976	13.52216
1978	12.33728
1979	12.02823
1980	11.09243
1981	10.9487
1982	11.59082
1983	12.10622
1984	13.1436
1985	12.66765
1986	14.36467
1987	13.6283
1988	12.92616
1989	13.31847
1990	13.00745
1991	11.92167
1992	12.56549
2000	12.46705
2004	12.82033
2008	17.21552
2009	12.87448