Year	per 1,000
1990	17.3
1991	16.7
1992	16.2
1993	15.6
1994	15.1
1995	14.6
1996	14.2
1997	13.7
1998	13.3
1999	12.9
2000	12.5
2001	12.2
2002	11.9
2003	11.6
2004	11.4
2005	11.1
2006	10.9
2007	10.6
2008	10.4
2009	10.1
2010	9.9
2011	9.7
2012	9.5
2013	9.4
2014	9.2
2015	9.1
2016	8.9
2017	8.7
2018	8.5
2019	8.3
2020	8.1
2021	7.9
2022	7.7