Year	Number of
1990	14.39
1991	14.23
1992	14.05
1993	13.86
1994	13.74
1995	13.62
1996	13.49
1997	13.36
1998	13.22
1999	13.36
2000	13.42
2001	13.36
2002	13.34
2003	13.38
2004	13.34
2005	13.15
2006	13.33
2007	13.29
2008	14.5
2009	16.1
2010	16.12
2011	15.97
2012	16.05
2013	16.46
2014	16.93
2015	17.4
2016	17.92