Year	Number of
1990	1.39
1991	1.43
1992	1.46
1993	1.5
1994	1.54
1995	1.59
1996	1.7
1997	1.83
1998	1.98
1999	2.17
2000	2.38
2001	2.43
2002	2.5
2003	2.57
2004	2.66
2005	2.73
2006	2.81
2007	2.87
2008	2.92
2009	2.96
2010	3.03
2011	3.09
2012	3.16
2013	3.25
2014	3.35
2015	3.43
2016	3.53