Year	per 1,000
1990	17.5
1991	17.1
1992	16.8
1993	16.5
1994	16.2
1995	15.9
1996	15.6
1997	15.3
1998	15.1
1999	14.8
2000	14.6
2001	14.4
2002	14.2
2003	14
2004	13.8
2005	13.6
2006	13.4
2007	13.2
2008	12.9
2009	12.7
2010	12.4
2011	12.1
2012	11.8
2013	11.5
2014	11.3
2015	11
2016	10.8
2017	10.5
2018	10.2
2019	10.1
2020	9.9
2021	9.8
2022	9.7