Year	per 1,000
1990	26.4
1991	26.1
1992	25.8
1993	25.5
1994	25.2
1995	24.8
1996	24.5
1997	24.1
1998	23.7
1999	23.2
2000	22.8
2001	22.3
2002	21.8
2003	21.3
2004	20.9
2005	20.5
2006	20
2007	19.6
2008	22.5
2009	18.9
2010	18.6
2011	18.3
2012	18
2013	17.7
2014	17.4
2015	17
2016	16.7
2017	16.4
2018	16.1
2019	15.8
2020	15.5
2021	15.2
2022	14.8