Year	per 1,000
1990	21.4
1991	21
1992	20.5
1993	20.1
1994	19.7
1995	19.2
1996	18.8
1997	18.4
1998	17.9
1999	17.5
2000	17.1
2001	16.7
2002	16.3
2003	15.9
2004	15.5
2005	15.1
2006	14.7
2007	14.3
2008	14
2009	13.6
2010	13.3
2011	13
2012	12.7
2013	12.4
2014	12.1
2015	11.8
2016	11.5
2017	11.2
2018	11
2019	10.8
2020	10.5
2021	10.3
2022	10.1