Year	Number
1970	19.18129
1971	18.94324
1972	18.94918
1973	20.09487
1974	19.75833
1975	19.43457
1976	19.51832
1977	19.53382
1979	19.77802
1980	19.5464
1981	20.73927
1982	20.77486
1983	20.34754
1984	19.96782
1985	19.10489
1986	18.06957
1987	18.38805
1988	17.90407
1994	15.3651
1997	15.99262
1998	21.43752
1999	23.79198
2001	23.86941
2002	23.71277
2003	21.65935
2004	19.62456
2005	19.29073
2006	18.53316
2007	17.65757
2008	19.92434