Year	Number of
1990	2.04
1991	2.08
1992	2.1
1993	2.15
1994	2.18
1995	2.23
1996	2.27
1997	2.31
1998	2.36
1999	2.46
2000	2.53
2001	2.58
2002	2.67
2003	2.76
2004	2.87
2005	2.95
2006	3.07
2007	3.18
2008	3.27
2009	3.36
2010	3.48
2011	3.58
2012	3.71
2013	3.85
2014	3.98
2015	4.1
2016	4.23