Year	Number of
1990	0.61
1991	0.66
1992	0.71
1993	0.77
1994	0.83
1995	0.89
1996	0.93
1997	0.97
1998	1
1999	1.04
2000	1.08
2001	1.12
2002	1.17
2003	1.21
2004	1.25
2005	1.3
2006	1.35
2007	1.39
2008	1.44
2009	1.49
2010	1.54
2011	1.6
2012	1.66
2013	1.72
2014	1.79
2015	1.86
2016	1.93