Year	Number of
1990	3.58
1991	3.67
1992	3.78
1993	3.91
1994	4.03
1995	4.14
1996	4.36
1997	4.5
1998	4.57
1999	4.61
2000	4.75
2001	4.75
2002	4.81
2003	4.82
2004	4.91
2005	4.89
2006	4.99
2007	5.06
2008	5.1
2009	5.27
2010	5.45
2011	5.55
2012	5.74
2013	5.8
2014	5.96
2015	6.14
2016	6.32