Year	Number of
1990	2.75
1991	2.84
1992	2.96
1993	3.09
1994	3.23
1995	3.37
1996	3.49
1997	3.58
1998	3.71
1999	3.83
2000	3.92
2001	4.07
2002	4.22
2003	4.34
2004	4.51
2005	4.72
2006	4.88
2007	5.06
2008	5.18
2009	5.28
2010	5.36
2011	5.41
2012	5.48
2013	5.62
2014	5.81
2015	6.05
2016	6.36