Year	Number of
1990	6.46
1991	6.38
1992	6.29
1993	6.23
1994	6.22
1995	6.2
1996	6.24
1997	6.23
1998	6.44
1999	6.24
2000	6.36
2001	6.01
2002	5.94
2003	5.85
2004	5.72
2005	5.67
2006	5.62
2007	5.49
2008	5.51
2009	5.56
2010	5.67
2011	5.78
2012	5.95
2013	6.21
2014	6.62
2015	6.79
2016	6.99