Year	Number of
1990	2.26
1991	2.3
1992	2.32
1993	2.29
1994	2.31
1995	2.33
1996	2.35
1997	2.34
1998	2.51
1999	2.48
2000	2.37
2001	2.36
2002	2.22
2003	2.25
2004	2.25
2005	2.31
2006	2.33
2007	2.36
2008	2.39
2009	2.41
2010	2.47
2011	2.5
2012	2.52
2013	2.6
2014	2.68
2015	2.75
2016	2.81