Year	per 1,000
1990	17.4
1991	17.2
1992	17
1993	16.8
1994	16.7
1995	16.5
1996	16.4
1997	16.2
1998	16
1999	15.8
2000	15.6
2001	15.3
2002	15.1
2003	14.8
2004	14.6
2005	14.3
2006	14
2007	13.7
2008	13.5
2009	13.2
2010	12.9
2011	12.6
2012	12.4
2013	12.1
2014	11.9
2015	11.6
2016	11.4
2017	11.1
2018	10.8
2019	10.6
2020	10.4
2021	10.1
2022	9.9