Year	per 1,000
1990	28.6
1991	27.8
1992	27.1
1993	26.3
1994	25.6
1995	24.9
1996	24.2
1997	23.5
1998	22.8
1999	22.1
2000	21.5
2001	20.9
2002	20.2
2003	19.6
2004	19.1
2005	18.5
2006	18
2007	17.6
2008	17.2
2009	16.9
2010	16.6
2011	16.5
2012	16.4
2013	16.4
2014	16.4
2015	16.4
2016	16.3
2017	16.2
2018	16.1
2019	15.9
2020	15.6
2021	15.3
2022	14.9