Year	per 1,000
1990	5.1
1991	4.8
1992	4.7
1993	4.8
1994	4.8
1995	4.8
1996	4.7
1997	4.4
1998	4.1
1999	3.9
2000	3.7
2001	3.6
2002	3.5
2003	3.4
2004	3.3
2005	3.2
2006	3.2
2007	3.5
2008	3.9
2009	4.1
2010	4
2011	3.9
2012	3.6
2013	3.3
2014	3.1
2015	2.9
2016	2.8
2017	2.7
2018	2.6
2019	2.7
2020	2.8
2021	2.8
2022	2.8