Year	per 1,000
1990	36.6
1991	36.6
1992	36.5
1993	36.5
1994	36.4
1995	36.3
1996	36.1
1997	35.8
1998	35.5
1999	35
2000	34.5
2001	33.8
2002	33.1
2003	32.2
2004	31.3
2005	30.3
2006	29.2
2007	28.1
2008	27
2009	25.9
2010	24.9
2011	23.8
2012	22.8
2013	22
2014	21.2
2015	20.6
2016	20
2017	19.5
2018	19
2019	18.5
2020	18.1
2021	17.7
2022	17.4