Year	per 1,000
1990	19.6
1991	19.4
1992	19.2
1993	19.1
1994	18.9
1995	18.8
1996	20.9
1997	18.6
1998	18.6
1999	18.6
2000	18.6
2001	18.6
2002	18.5
2003	18.5
2004	18.4
2005	18.3
2006	18.4
2007	18.5
2008	18.7
2009	19
2010	19.2
2011	19.4
2012	19.6
2013	19.7
2014	19.6
2015	19.3
2016	19
2017	18.5
2018	18.1
2019	17.6
2020	17.2
2021	16.8
2022	16.4