Year	Number of
1990	2.48
1991	2.61
1992	2.72
1993	2.88
1994	3.02
1995	3.19
1996	3.34
1997	3.49
1998	3.66
1999	3.84
2000	4.02
2001	4.22
2002	4.44
2003	4.67
2004	4.9
2005	5.17
2006	5.42
2007	5.57
2008	5.71
2009	5.84
2010	5.98
2011	6.08
2012	6.2
2013	6.32
2014	6.42
2015	6.53
2016	6.76