Year	Number of
1990	2.39
1991	2.49
1992	2.61
1993	2.73
1994	2.87
1995	3
1996	3.12
1997	3.19
1998	3.27
1999	3.41
2000	3.58
2001	3.7
2002	3.81
2003	3.89
2004	3.92
2005	4.01
2006	4.09
2007	4.14
2008	4.32
2009	4.56
2010	4.82
2011	5.01
2012	5.2
2013	5.35
2014	5.53
2015	5.78
2016	6.06