Year	Number of
1990	2.83
1991	2.88
1992	2.91
1993	2.81
1994	2.79
1995	2.89
1996	2.82
1997	2.86
1998	2.86
1999	2.87
2000	2.91
2001	3
2002	3.08
2003	3.21
2004	3.25
2005	3.39
2006	3.52
2007	3.56
2008	3.6
2009	3.66
2010	3.8
2011	3.91
2012	4.05
2013	4.13
2014	4.17
2015	4.24
2016	4.35