Year	per 1,000
1990	5.9
1991	6
1992	6.1
1993	6.2
1994	6.2
1995	6.2
1996	6.1
1997	5.9
1998	5.7
1999	5.5
2000	5.2
2001	4.9
2002	4.7
2003	4.4
2004	4.2
2005	4
2006	3.9
2007	3.8
2008	3.7
2009	3.6
2010	3.5
2011	3.5
2012	3.5
2013	3.5
2014	3.5
2015	3.5
2016	3.5
2017	3.4
2018	3.3
2019	3.2
2020	3.1
2021	3
2022	2.9