Year	Number of
1990	1.61
1991	1.67
1992	1.74
1993	1.82
1994	1.89
1995	1.97
1996	2.01
1997	2.04
1998	2.07
1999	2.1
2000	2.13
2001	2.17
2002	2.22
2003	2.25
2004	2.29
2005	2.33
2006	2.36
2007	2.39
2008	2.41
2009	2.43
2010	2.45
2011	2.46
2012	2.46
2013	2.48
2014	2.5
2015	2.53
2016	2.56