Year	per 1,000
1990	18.1
1991	17.7
1992	17.3
1993	16.8
1994	16.4
1995	16
1996	15.6
1997	15.2
1998	14.8
1999	14.4
2000	14
2001	13.6
2002	13.2
2003	12.8
2004	12.5
2005	12.1
2006	11.7
2007	11.4
2008	11.1
2009	10.8
2010	10.4
2011	10.2
2012	9.9
2013	9.6
2014	9.3
2015	9.1
2016	8.8
2017	8.6
2018	8.4
2019	8.2
2020	8
2021	7.8
2022	7.6