Year	per 1,000
1990	5.7
1991	5.6
1992	5.5
1993	5.5
1994	5.4
1995	5.3
1996	5.3
1997	5.2
1998	6.7
1999	5.1
2000	5.1
2001	5
2002	4.9
2003	4.9
2004	4.8
2005	4.7
2006	4.7
2007	4.6
2008	4.5
2009	4.5
2010	4.4
2011	4.3
2012	4.2
2013	4.2
2014	4.1
2015	4
2016	3.9
2017	3.8
2018	3.8
2019	3.7
2020	3.6
2021	3.5
2022	3.4