Year	per 1,000
1990	12.2
1991	11.9
1992	11.6
1993	11.3
1994	11
1995	10.6
1996	10.3
1997	10
1998	11
1999	9.5
2000	9.2
2001	8.9
2002	8.7
2003	8.5
2004	8.3
2005	8
2006	7.9
2007	7.7
2008	7.5
2009	7.3
2010	7.2
2011	7
2012	6.9
2013	6.7
2014	6.6
2015	6.5
2016	6.4
2017	6.3
2018	6.1
2019	6
2020	5.9
2021	5.8
2022	5.7