Year	Number of
1990	2.29
1991	2.38
1992	2.48
1993	2.57
1994	2.67
1995	2.77
1996	2.86
1997	2.94
1998	3.02
1999	3.1
2000	3.19
2001	3.3
2002	3.43
2003	3.57
2004	3.73
2005	3.9
2006	4.07
2007	4.25
2008	4.45
2009	4.66
2010	4.91
2011	5.11
2012	5.33
2013	5.59
2014	5.85
2015	6.12
2016	6.41