Year	Number of
1990	13.39
1991	13.55
1992	13.74
1993	13.61
1994	13.45
1995	13.24
1996	12.73
1997	12.29
1998	12.56
1999	13.06
2000	13.78
2001	14.06
2002	14.6
2003	14.65
2004	14.82
2005	15.32
2006	15.69
2007	15.95
2008	16.24
2009	16.59
2010	16.87
2011	17.26
2012	17.62
2013	17.88
2014	18.19
2015	18.53
2016	18.95