Year	Number of
1990	3.15
1991	3.2
1992	3.26
1993	3.33
1994	3.4
1995	3.46
1996	3.48
1997	3.5
1998	3.53
1999	3.55
2000	3.55
2001	3.59
2002	3.65
2003	3.72
2004	3.79
2005	3.86
2006	3.9
2007	3.98
2008	4.05
2009	4.17
2010	4.3
2011	4.45
2012	4.52
2013	4.6
2014	4.66
2015	4.73
2016	4.71