Year	per 1,000
1990	19
1991	18.8
1992	18.7
1993	18.6
1994	18.5
1995	18.4
1996	18.3
1997	18.1
1998	17.9
1999	17.6
2000	17.3
2001	17
2002	16.7
2003	16.3
2004	16
2005	15.6
2006	15.3
2007	14.9
2008	14.5
2009	14.2
2010	13.8
2011	13.5
2012	13.2
2013	12.9
2014	12.6
2015	12.3
2016	12
2017	11.8
2018	11.5
2019	11.2
2020	10.9
2021	10.7
2022	10.5