Year	Number of
1990	1.7
1991	1.74
1992	1.79
1993	1.85
1994	1.91
1995	1.97
1996	2.02
1997	2.07
1998	2.13
1999	2.2
2000	2.28
2001	2.31
2002	2.35
2003	2.4
2004	2.46
2005	2.51
2006	2.54
2007	2.58
2008	2.62
2009	2.66
2010	2.71
2011	2.74
2012	2.79
2013	2.82
2014	2.86
2015	2.91
2016	2.97