Year	per 1,000
1990	5.6
1991	5.6
1992	5.6
1993	5.6
1994	5.6
1995	5.5
1996	5.4
1997	5.2
1998	5.1
1999	4.9
2000	4.8
2001	4.7
2002	4.5
2003	4.4
2004	4.3
2005	6
2006	4.3
2007	4.3
2008	4.2
2009	4.2
2010	4.2
2011	4.2
2012	4.1
2013	4.1
2014	4
2015	4
2016	3.9
2017	3.8
2018	3.8
2019	3.7
2020	3.6
2021	3.5
2022	3.4