Year	Number of
1990	2.18
1991	2.25
1992	2.31
1993	2.37
1994	2.45
1995	2.53
1996	2.57
1997	2.63
1998	2.7
1999	2.76
2000	2.83
2001	2.88
2002	2.93
2003	2.98
2004	3.04
2005	3.08
2006	3.14
2007	3.18
2008	3.23
2009	3.28
2010	3.34
2011	3.41
2012	3.49
2013	3.56
2014	3.64
2015	3.72
2016	3.81