Year	Number of
1990	1.16
1991	1.19
1992	1.21
1993	1.24
1994	1.27
1995	1.3
1996	1.31
1997	1.33
1998	1.35
1999	1.36
2000	1.37
2001	1.4
2002	1.44
2003	1.47
2004	1.51
2005	1.54
2006	1.56
2007	1.59
2008	1.62
2009	1.66
2010	1.7
2011	1.75
2012	1.79
2013	1.83
2014	1.86
2015	1.9
2016	1.93