Year	per 1,000
1990	37.8
1991	32.5
1992	38
1993	32.4
1994	32.3
1995	32.1
1996	31.8
1997	31.4
1998	30.9
1999	30.2
2000	29.4
2001	28.5
2002	27.4
2003	26.2
2004	24.9
2005	23.6
2006	22.2
2007	20.8
2008	19.4
2009	18
2010	16.8
2011	15.6
2012	14.6
2013	13.6
2014	12.8
2015	12.2
2016	11.6
2017	11.1
2018	10.6
2019	10.2
2020	9.8
2021	9.4
2022	9.1