Year	per 1,000
1990	12
1991	12
1992	11.9
1993	11.8
1994	11.8
1995	11.7
1996	11.6
1997	11.5
1998	11.3
1999	11.2
2000	11
2001	10.8
2002	10.5
2003	10.3
2004	10.1
2005	9.8
2006	9.6
2007	9.4
2008	9.1
2009	8.9
2010	8.6
2011	8.4
2012	8.2
2013	8
2014	7.8
2015	7.6
2016	7.4
2017	7.2
2018	7
2019	6.9
2020	6.7
2021	6.5
2022	6.3