Year	Number of
1990	0.58
1991	0.61
1992	0.64
1993	0.67
1994	0.7
1995	0.73
1996	0.76
1997	0.8
1998	0.83
1999	0.88
2000	0.92
2001	0.97
2002	1.02
2003	1.09
2004	1.15
2005	1.22
2006	1.27
2007	1.33
2008	1.38
2009	1.44
2010	1.5
2011	1.55
2012	1.6
2013	1.66
2014	1.72
2015	1.78
2016	1.88