Year	per 1,000
1990	10.4
1991	10.9
1992	11.4
1993	11.8
1994	11.8
1995	11.6
1996	11.2
1997	10.6
1998	9.8
1999	8.9
2000	8.2
2001	7.6
2002	7.3
2003	7.2
2004	7.3
2005	7.3
2006	7.3
2007	7.2
2008	7
2009	6.7
2010	6.5
2011	6.2
2012	6
2013	5.7
2014	5.4
2015	5.2
2016	4.9
2017	4.7
2018	4.6
2019	4.5
2020	4.5
2021	4.5
2022	4.5