Year	per 1,000
1990	25.6
1991	25.2
1992	24.8
1993	24.4
1994	24
1995	23.5
1996	23
1997	22.6
1998	22.1
1999	21.6
2000	21.1
2001	20.6
2002	20.1
2003	19.6
2004	19.1
2005	18.6
2006	18.1
2007	17.7
2008	17.3
2009	16.9
2010	16.5
2011	16.1
2012	15.7
2013	15.4
2014	15
2015	14.7
2016	14.5
2017	14.2
2018	13.9
2019	13.6
2020	13.3
2021	12.9
2022	12.6