Year	per 1,000
1990	23.4
1991	23.1
1992	22.9
1993	22.8
1994	22.8
1995	22.8
1996	22.8
1997	22.8
1998	22.8
1999	22.8
2000	22.8
2001	22.8
2002	22.8
2003	22.8
2004	22.8
2005	22.8
2006	22.7
2007	22.5
2008	22.2
2009	21.8
2010	21.3
2011	20.8
2012	20.3
2013	19.9
2014	19.4
2015	19
2016	18.6
2017	18.2
2018	17.8
2019	17.4
2020	17
2021	16.6
2022	17.7