Year	per 1,000
1990	23.5
1991	22.9
1992	22.6
1993	22.5
1994	22.7
1995	22.9
1996	23.2
1997	23.3
1998	23.2
1999	22.8
2000	22
2001	21
2002	19.8
2003	18.5
2004	17.2
2005	15.9
2006	14.8
2007	13.7
2008	12.7
2009	11.8
2010	10.9
2011	10.1
2012	9.3
2013	8.5
2014	7.7
2015	7
2016	6.4
2017	5.8
2018	5.3
2019	4.9
2020	4.6
2021	4.3
2022	4.1