Year	per 1,000
1990	17.8
1991	17.9
1992	17.9
1993	17.9
1994	17.9
1995	17.9
1996	17.9
1997	17.9
1998	17.9
1999	17.9
2000	17.9
2001	17.9
2002	17.8
2003	17.7
2004	17.6
2005	17.3
2006	16.9
2007	16.5
2008	16.1
2009	15.7
2010	15.2
2011	14.8
2012	14.4
2013	14
2014	13.6
2015	13.2
2016	12.7
2017	13.6
2018	12.1
2019	11.7
2020	11.3
2021	12.1
2022	10.7