Year	Number
1971	10.63556
1981	9.13867
1982	9.2153
1983	9.0083
1984	8.92738
1985	8.49326
1986	8.06186
1987	8.01044
1988	7.97481
1989	9.53343
1990	9.48317
1991	9.12324
1992	8.836
1993	8.25819
1994	8.18068
1999	7.73215
2000	7.70686
2001	7.73718
2002	7.98662
2003	8.15718
2004	8.26779
2005	8.43618
2006	8.32971
2007	8.50271
2008	8.63742
2009	8.74491
2010	8.7574
2011	9.01237
2012	9.3458