Year	per 1,000
1990	20.3
1991	20
1992	19.6
1993	19.3
1994	18.9
1995	18.5
1996	18
1997	17.5
1998	16.9
1999	16.4
2000	15.9
2001	15.3
2002	14.8
2003	14.3
2004	13.9
2005	13.4
2006	13
2007	12.6
2008	12.2
2009	11.8
2010	11.3
2011	10.9
2012	10.6
2013	10.2
2014	9.9
2015	9.6
2016	9.3
2017	9
2018	8.8
2019	8.5
2020	8.3
2021	8
2022	9.6