Year	Number of
1990	2.26
1991	2.33
1992	2.41
1993	2.45
1994	2.47
1995	2.52
1996	2.54
1997	2.59
1998	2.62
1999	2.67
2000	2.71
2001	2.8
2002	2.91
2003	3.04
2004	3.18
2005	3.33
2006	3.46
2007	3.56
2008	3.66
2009	3.76
2010	3.85
2011	3.96
2012	4.06
2013	4.15
2014	4.26
2015	4.35
2016	4.57