Year	Number of
1990	19.16
1991	19.61
1992	20.53
1993	20.95
1994	21.47
1995	21.56
1996	21.47
1997	21.45
1998	21.29
1999	21.05
2000	21.22
2001	20.81
2002	20.21
2003	20.44
2004	20.67
2005	21.34
2006	21.38
2007	21.21
2008	21.13
2009	21.41
2010	21.44
2011	21.71
2012	22.27
2013	22.78
2014	23.44
2015	24.17
2016	24.83