Year	Number of
1990	2.67
1991	2.65
1992	2.63
1993	2.54
1994	2.61
1995	2.57
1996	2.53
1997	2.45
1998	2.43
1999	2.47
2000	2.49
2001	2.46
2002	2.48
2003	2.44
2004	2.38
2005	2.29
2006	2.26
2007	2.15
2008	2.2
2009	2.22
2010	2.28
2011	2.3
2012	2.31
2013	2.35
2014	2.54
2015	2.66
2016	2.79