Year	per 1,000
1990	18.1
1991	17.7
1992	17.4
1993	17.1
1994	16.7
1995	16.4
1996	16
1997	15.6
1998	15.2
1999	14.8
2000	14.4
2001	14
2002	13.6
2003	13.2
2004	12.7
2005	12.4
2006	12
2007	11.7
2008	11.4
2009	11.1
2010	10.9
2011	10.7
2012	10.6
2013	10.5
2014	10.4
2015	10.3
2016	10.2
2017	10.2
2018	10
2019	9.9
2020	9.6
2021	9.4
2022	10