Year	per 1,000
1990	4.2
1991	4.6
1992	4.5
1993	4.2
1994	3.7
1995	3.4
1996	3.4
1997	3.8
1998	4.2
1999	4.3
2000	4.1
2001	3.8
2002	3.5
2003	3.3
2004	3.3
2005	3.4
2006	3.4
2007	3.4
2008	3.2
2009	3
2010	2.8
2011	2.6
2012	2.5
2013	2.4
2014	2.4
2015	2.3
2016	2.2
2017	2.2
2018	2.2
2019	2.2
2020	2.3
2021	2.4
2022	2.4