Year	Number of
1990	3.25
1991	3.31
1992	3.38
1993	3.46
1994	3.54
1995	3.63
1996	3.76
1997	3.89
1998	4.03
1999	4.17
2000	4.32
2001	4.39
2002	4.46
2003	4.54
2004	4.62
2005	4.7
2006	4.73
2007	4.76
2008	4.79
2009	4.82
2010	4.85
2011	4.94
2012	5.03
2013	5.12
2014	5.22
2015	5.32
2016	5.45