Year	per 1,000
1990	20.8
1991	20.4
1992	20
1993	19.6
1994	19.2
1995	18.8
1996	18.4
1997	17.9
1998	17.6
1999	17.2
2000	16.8
2001	16.4
2002	16.1
2003	15.7
2004	15.4
2005	15
2006	14.7
2007	14.4
2008	14.1
2009	13.7
2010	13.4
2011	13.1
2012	12.8
2013	12.4
2014	12.1
2015	11.8
2016	11.4
2017	11.1
2018	10.8
2019	10.6
2020	10.5
2021	10.4
2022	10.2