Year	per 1,000
1990	6.8
1991	5.9
1992	5.7
1993	5.5
1994	5.3
1995	5.2
1996	5.1
1997	4.9
1998	4.8
1999	4.6
2000	4.5
2001	4.4
2002	4.4
2003	4.6
2004	4.2
2005	4.2
2006	4.2
2007	4.1
2008	4.1
2009	4.1
2010	4
2011	4.1
2012	4.6
2013	4.6
2014	4.5
2015	4.6
2016	4.4
2017	4.5
2018	4.6
2019	4.2
2020	3.9
2021	3.9
2022	3.8