Year	per 1,000
1990	19.4
1991	19.5
1992	19.4
1993	19.4
1994	19.2
1995	19
1996	18.7
1997	18.3
1998	17.9
1999	17.3
2000	16.7
2001	16
2002	15.4
2003	17
2004	14.2
2005	13.6
2006	13
2007	12.5
2008	12
2009	11.6
2010	11.2
2011	10.9
2012	10.7
2013	10.5
2014	11.9
2015	12.1
2016	10.1
2017	10
2018	9.8
2019	9.7
2020	9.4
2021	9.2
2022	9