Year	per 1,000
1990	18.3
1991	18.6
1992	19.1
1993	19.6
1994	20.2
1995	20.7
1996	21.2
1997	21.5
1998	21.5
1999	21.2
2000	20.5
2001	19.6
2002	18.6
2003	17.6
2004	16.6
2005	15.6
2006	14.7
2007	13.9
2008	13.2
2009	12.5
2010	11.8
2011	11.2
2012	10.6
2013	10
2014	9.5
2015	9
2016	8.5
2017	8.1
2018	7.8
2019	7.4
2020	7.2
2021	7
2022	6.8