Year	per 1,000
1990	4.6
1991	3.8
1992	3.6
1993	3.4
1994	3.3
1995	3.2
1996	3
1997	2.9
1998	2.8
1999	2.7
2000	2.7
2001	2.6
2002	2.5
2003	2.8
2004	2.4
2005	2.4
2006	2.4
2007	2.4
2008	2.4
2009	2.3
2010	2.3
2011	2.3
2012	2.5
2013	2.5
2014	2.4
2015	2.4
2016	2.3
2017	2.2
2018	2.2
2019	2
2020	1.9
2021	1.9
2022	1.8