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