Year	Number of
1990	2.58
1991	2.69
1992	2.79
1993	2.85
1994	2.9
1995	3.06
1996	3.13
1997	3.25
1998	3.49
1999	3.54
2000	3.5
2001	3.58
2002	3.47
2003	3.51
2004	3.54
2005	3.67
2006	3.86
2007	4.11
2008	4.38
2009	4.49
2010	4.69
2011	4.84
2012	5.02
2013	5.19
2014	5.38
2015	5.57
2016	5.74