Year	Number of
1990	2.2
1991	2.33
1992	2.42
1993	2.38
1994	2.4
1995	2.52
1996	2.46
1997	2.53
1998	2.47
1999	2.41
2000	2.37
2001	2.38
2002	2.4
2003	2.44
2004	2.4
2005	2.46
2006	2.6
2007	2.67
2008	2.74
2009	2.86
2010	3
2011	3.17
2012	3.29
2013	3.48
2014	3.54
2015	3.65
2016	3.86