Year	Number
1980	12.36024
1981	12.3231
1982	12.3007
1983	12.26512
1984	12.24029
1985	12.19845
1986	12.21131
1987	12.12707
1988	12.04519
1989	11.98043
1990	11.8115
1991	11.66954
1992	11.59966
1993	11.55624
1994	11.57793
1995	11.65361
1996	11.57203
1997	11.58785
1998	11.69391
1999	11.7544
2000	11.67828
2001	11.67945
2002	11.73312
2003	11.76852
2004	11.60135
2005	11.4976
2006	11.35737
2007	11.1813
2008	11.12936
2009	11.11394
2010	11.18062
2011	11.14743
2012	11.10405
2013	11.51975
2014	11.44363
2015	11.29639
2016	11.09253
2017	11.08297
2018	11.08115