Year	per 1,000
1990	21.3
1991	20.5
1992	19.7
1993	18.9
1994	18.1
1995	17.3
1996	16.6
1997	15.9
1998	15.2
1999	14.5
2000	13.8
2001	13.2
2002	12.6
2003	12
2004	11.4
2005	10.9
2006	10.4
2007	9.9
2008	9.4
2009	8.9
2010	8.5
2011	8.1
2012	7.8
2013	7.4
2014	7.1
2015	6.7
2016	6.4
2017	6.2
2018	5.9
2019	5.6
2020	5.4
2021	5.2
2022	5