Year	per 1,000
1990	6.8
1991	6.3
1992	5.9
1993	5.9
1994	5.9
1995	6
1996	6
1997	5.8
1998	5.6
1999	5.3
2000	5.2
2001	5.1
2002	5.1
2003	5
2004	4.7
2005	4.5
2006	4.3
2007	4.1
2008	4.1
2009	4
2010	4
2011	3.9
2012	3.7
2013	3.7
2014	3.6
2015	3.6
2016	3.7
2017	3.6
2018	3.5
2019	3.3
2020	3.3
2021	3.4
2022	3.5