Year	Number of
1990	3.34
1991	3.38
1992	3.45
1993	3.53
1994	3.61
1995	3.69
1996	3.73
1997	3.68
1998	3.64
1999	3.68
2000	3.75
2001	3.75
2002	3.74
2003	3.69
2004	3.59
2005	3.58
2006	3.56
2007	3.48
2008	3.53
2009	3.67
2010	3.8
2011	3.83
2012	3.87
2013	3.85
2014	3.86
2015	3.92
2016	4.01