Year	Number of
1990	2.58
1991	2.71
1992	2.83
1993	2.95
1994	3.06
1995	3.16
1996	3.25
1997	3.34
1998	3.42
1999	3.52
2000	3.61
2001	3.72
2002	3.85
2003	3.99
2004	4.14
2005	4.31
2006	4.5
2007	4.7
2008	4.9
2009	5.12
2010	5.37
2011	5.59
2012	5.83
2013	6.12
2014	6.41
2015	6.7
2016	6.96