Year	Number of
1990	0.91
1991	0.94
1992	0.98
1993	1.02
1994	1.06
1995	1.11
1996	1.14
1997	1.19
1998	1.25
1999	1.22
2000	1.26
2001	1.26
2002	1.28
2003	1.33
2004	1.42
2005	1.51
2006	1.58
2007	1.65
2008	1.74
2009	1.87
2010	1.94
2011	2.03
2012	2.13
2013	2.23
2014	2.33
2015	2.42
2016	2.49