Year	per 1,000
1990	22.1
1991	21.9
1992	21.8
1993	21.6
1994	21.5
1995	21.3
1996	21.1
1997	21
1998	20.8
1999	20.6
2000	20.3
2001	20.1
2002	19.8
2003	19.5
2004	19.2
2005	18.9
2006	18.6
2007	18.2
2008	17.9
2009	17.5
2010	17.1
2011	16.7
2012	16.3
2013	15.9
2014	15.5
2015	15.2
2016	14.8
2017	14.5
2018	14.1
2019	13.7
2020	13.3
2021	12.9
2022	12.6