Year	Number of
1990	1.72
1991	1.81
1992	1.92
1993	2.04
1994	2.17
1995	2.31
1996	2.37
1997	2.43
1998	2.5
1999	2.62
2000	2.67
2001	2.71
2002	2.7
2003	2.78
2004	2.88
2005	3.02
2006	3.15
2007	3.32
2008	3.47
2009	3.69
2010	3.93
2011	4.12
2012	4.29
2013	4.49
2014	4.69
2015	4.87
2016	4.99