Year	per 1,000
1990	18.5
1991	15.7
1992	18.5
1993	15.7
1994	15.6
1995	15.6
1996	15.5
1997	15.3
1998	15.1
1999	14.9
2000	14.6
2001	14.3
2002	13.9
2003	13.4
2004	12.9
2005	12.4
2006	11.9
2007	11.3
2008	10.7
2009	10.2
2010	9.7
2011	9.2
2012	8.7
2013	8.3
2014	7.9
2015	7.6
2016	7.3
2017	7.1
2018	6.8
2019	6.6
2020	6.4
2021	6.2
2022	6.1