Year	Number of
1990	17.45
1991	17.56
1992	17.6
1993	17.73
1994	17.94
1995	18.27
1996	18.6
1997	18.86
1998	18.99
1999	19.16
2000	19.33
2001	19.42
2002	19.49
2003	19.61
2004	19.75
2005	19.83
2006	19.87
2007	19.81
2008	19.88
2009	20.07
2010	20.32
2011	20.23
2012	21.42
2013	21.6
2014	21.95
2015	22.36
2016	22.82