Year	Number of
1990	0.5
1991	0.52
1992	0.55
1993	0.58
1994	0.61
1995	0.64
1996	0.66
1997	0.67
1998	0.69
1999	0.7
2000	0.75
2001	0.79
2002	0.83
2003	0.86
2004	0.9
2005	0.97
2006	1.01
2007	1.04
2008	1.08
2009	1.11
2010	1.14
2011	1.19
2012	1.23
2013	1.27
2014	1.31
2015	1.35
2016	1.39