Year	per 1,000
1990	4.1
1991	4
1992	3.8
1993	3.7
1994	3.6
1995	3.5
1996	3.4
1997	3.4
1998	3.3
1999	3.2
2000	3.1
2001	3.1
2002	3
2003	2.9
2004	2.8
2005	2.7
2006	2.6
2007	2.5
2008	2.4
2009	6.2
2010	2.2
2011	2.1
2012	2
2013	1.9
2014	1.7
2015	1.6
2016	1.6
2017	1.5
2018	1.4
2019	1.3
2020	1.3
2021	1.2
2022	1.2