Year	Number of
1990	5.38
1991	5.58
1992	5.84
1993	5.97
1994	6.06
1995	6.17
1996	6.04
1997	6
1998	5.95
1999	5.97
2000	5.94
2001	6.04
2002	6.12
2003	6.17
2004	6.11
2005	6.3
2006	6.39
2007	6.51
2008	6.65
2009	6.87
2010	7.07
2011	7.17
2012	7.29
2013	7.55
2014	7.89
2015	8.02
2016	8.55