Year	per 1,000
1990	34.3
1991	32
1992	29.8
1993	27.5
1994	25.5
1995	23.6
1996	21.8
1997	20
1998	18.4
1999	16.9
2000	15.6
2001	14.3
2002	13.3
2003	12.4
2004	11.7
2005	11.2
2006	11
2007	11
2008	11.3
2009	11.8
2010	12.6
2011	13.5
2012	14.6
2013	15.7
2014	16.7
2015	17.5
2016	18
2017	18.2
2018	18.2
2019	18.1
2020	18
2021	17.8
2022	17.6