Year	Number of
1990	13.39
1991	13.02
1992	12.76
1993	12.52
1994	12.26
1995	12.01
1996	12.27
1997	12.68
1998	13.24
1999	13.54
2000	13.87
2001	14.17
2002	14.63
2003	15.12
2004	15.34
2005	15.62
2006	15.46
2007	15.61
2008	15.43
2009	15.83
2010	16.17
2011	16.76
2012	17.19
2013	17.48
2014	17.94
2015	18.25
2016	18.53