Year	per 1,000
1990	25.8
1991	25.2
1992	24.9
1993	24.7
1994	24.7
1995	24.7
1996	24.7
1997	24.7
1998	24.7
1999	24.7
2000	24.7
2001	24.7
2002	24.7
2003	24.7
2004	24.7
2005	24.7
2006	24.6
2007	24.3
2008	23.8
2009	23
2010	25
2011	24.2
2012	20.8
2013	20.1
2014	19.4
2015	18.8
2016	18.2
2017	17.5
2018	16.9
2019	16.4
2020	15.8
2021	15.3
2022	14.8