Year	Number of
1990	2.61
1991	2.65
1992	2.71
1993	2.76
1994	2.81
1995	2.86
1996	2.84
1997	2.83
1998	2.8
1999	2.77
2000	2.73
2001	2.73
2002	2.74
2003	2.74
2004	2.75
2005	2.77
2006	2.8
2007	2.86
2008	2.9
2009	2.91
2010	2.97
2011	2.97
2012	2.94
2013	2.95
2014	3
2015	3.15
2016	3.19