Year	Number of
1990	1.77
1991	1.85
1992	1.93
1993	2.01
1994	2.09
1995	2.17
1996	2.21
1997	2.23
1998	2.26
1999	2.28
2000	2.32
2001	2.35
2002	2.39
2003	2.43
2004	2.47
2005	2.52
2006	2.53
2007	2.55
2008	2.56
2009	2.56
2010	2.57
2011	2.58
2012	2.6
2013	2.64
2014	2.68
2015	2.73
2016	2.79