Year	Number of
1990	3.88
1991	3.96
1992	4.01
1993	4.04
1994	4.1
1995	4.18
1996	4.21
1997	4.24
1998	4.34
1999	4.39
2000	4.22
2001	4.18
2002	4.36
2003	4.6
2004	4.87
2005	4.97
2006	4.97
2007	4.94
2008	4.87
2009	4.96
2010	5.03
2011	5.18
2012	5.28
2013	5.45
2014	5.62
2015	5.79
2016	5.95