Year	per 1,000
1990	20.6
1991	19.9
1992	19.1
1993	18.3
1994	17.6
1995	16.8
1996	16.1
1997	15.3
1998	14.6
1999	14
2000	13.3
2001	12.7
2002	12
2003	11.4
2004	10.7
2005	10.1
2006	9.4
2007	8.7
2008	8
2009	7.4
2010	6.8
2011	6.2
2012	5.6
2013	5.2
2014	4.7
2015	4.3
2016	3.9
2017	3.6
2018	3.4
2019	3.2
2020	3.1
2021	2.9
2022	2.8