Year	per 1,000
1990	20.5
1991	20.3
1992	20.2
1993	20.2
1994	20.2
1995	20.2
1996	20.2
1997	20.3
1998	20.4
1999	20.5
2000	20.5
2001	20.5
2002	20.4
2003	20.3
2004	20.2
2005	20
2006	19.8
2007	19.6
2008	19.3
2009	19.1
2010	18.8
2011	18.6
2012	18.4
2013	18.1
2014	18
2015	17.8
2016	17.6
2017	17.3
2018	17.1
2019	16.9
2020	16.8
2021	16.6
2022	16.4