Year	per 1,000
1990	5.8
1991	6.2
1992	6.6
1993	6.8
1994	6.9
1995	6.8
1996	6.6
1997	6.2
1998	5.7
1999	5.2
2000	4.8
2001	4.5
2002	4.4
2003	4.3
2004	4.3
2005	4.3
2006	4.3
2007	4.1
2008	4
2009	3.9
2010	3.8
2011	3.8
2012	3.8
2013	3.7
2014	3.7
2015	3.7
2016	3.6
2017	3.6
2018	3.5
2019	3.5
2020	3.5
2021	3.5
2022	3.5