Year	Number of
1990	2.47
1991	2.55
1992	2.61
1993	2.67
1994	2.68
1995	2.72
1996	2.7
1997	2.6
1998	2.59
1999	2.45
2000	2.42
2001	2.27
2002	2.25
2003	2.21
2004	2.16
2005	2.19
2006	2.18
2007	2.16
2008	2.15
2009	2.17
2010	2.26
2011	2.27
2012	2.36
2013	2.46
2014	2.54
2015	2.6
2016	2.76