Year	per 1,000
1990	17.5
1991	17.2
1992	17
1993	16.7
1994	16.3
1995	15.9
1996	15.5
1997	15
1998	14.5
1999	13.9
2000	13.4
2001	12.8
2002	12.3
2003	11.9
2004	11.4
2005	11
2006	10.6
2007	10.2
2008	9.8
2009	9.4
2010	9
2011	8.7
2012	8.3
2013	8.1
2014	7.8
2015	7.6
2016	7.4
2017	7.2
2018	7
2019	6.8
2020	6.6
2021	6.5
2022	6.9