Year	per 1,000
1990	11.3
1991	11.1
1992	11
1993	11.1
1994	13.5
1995	10.4
1996	10.2
1997	10
1998	9.8
1999	9.6
2000	9.4
2001	9.1
2002	8.9
2003	8.7
2004	8.5
2005	8.3
2006	8.1
2007	7.9
2008	7.8
2009	7.5
2010	7.4
2011	7.2
2012	7
2013	6.8
2014	6.7
2015	6.5
2016	6.4
2017	6.3
2018	6.1
2019	6
2020	5.9
2021	5.8
2022	5.7