Year	Number of
1990	3.53
1991	3.73
1992	3.87
1993	4.11
1994	4.26
1995	4.46
1996	4.13
1997	3.94
1998	3.83
1999	3.86
2000	3.88
2001	3.72
2002	4.13
2003	4.44
2004	4.63
2005	4.76
2006	4.98
2007	5.21
2008	5.69
2009	6.12
2010	6.53
2011	6.96
2012	7.5
2013	7.84
2014	8.61
2015	9.19
2016	9.92