Year	Number of
1990	4.02
1991	4.34
1992	4.63
1993	4.72
1994	4.7
1995	4.61
1996	4.54
1997	4.49
1998	4.45
1999	4.38
2000	4.33
2001	4.3
2002	4.3
2003	4.56
2004	4.97
2005	5.21
2006	5.38
2007	5.61
2008	5.76
2009	5.92
2010	6.06
2011	6.21
2012	6.36
2013	6.47
2014	6.61
2015	6.73
2016	6.73