Year	per 1,000
1990	6.1
1991	6
1992	5.9
1993	5.8
1994	5.8
1995	5.7
1996	5.6
1997	5.6
1998	5.5
1999	5.5
2000	5.4
2001	5.4
2002	5.4
2003	5.3
2004	5.3
2005	5.3
2006	5.2
2007	5.2
2008	5.1
2009	5.1
2010	5
2011	4.9
2012	4.8
2013	4.8
2014	4.7
2015	4.6
2016	4.5
2017	4.5
2018	4.4
2019	4.3
2020	4.2
2021	4.1
2022	4.1