Year	Number of
1990	2.51
1991	2.62
1992	2.73
1993	2.85
1994	2.98
1995	3.11
1996	3.21
1997	3.32
1998	3.42
1999	3.51
2000	3.61
2001	3.71
2002	3.85
2003	4.01
2004	4.19
2005	4.38
2006	4.56
2007	4.74
2008	4.94
2009	5.17
2010	5.42
2011	5.63
2012	5.89
2013	6.17
2014	6.45
2015	6.71
2016	6.97