Year	Number of
1990	1.57
1991	1.58
1992	1.59
1993	1.61
1994	1.63
1995	1.66
1996	1.63
1997	1.6
1998	1.56
1999	1.52
2000	1.49
2001	1.49
2002	1.48
2003	1.46
2004	1.44
2005	1.42
2006	1.43
2007	1.45
2008	1.47
2009	1.49
2010	1.51
2011	1.56
2012	1.61
2013	1.67
2014	1.73
2015	1.8
2016	1.87