Year	per 1,000
1990	9.7
1991	9.4
1992	9
1993	8.7
1994	8.3
1995	8
1996	7.6
1997	7.3
1998	7
1999	6.8
2000	6.5
2001	6.3
2002	6.1
2003	5.9
2004	5.8
2005	5.6
2006	5.5
2007	5.3
2008	5.1
2009	5
2010	4.9
2011	4.8
2012	4.7
2013	4.6
2014	4.5
2015	4.5
2016	4.4
2017	4.4
2018	4.3
2019	4.3
2020	4.2
2021	4.2
2022	4.2