Year	per 1,000
1990	23.8
1991	22.9
1992	23
1993	23.4
1994	37.4
1995	22.9
1996	23
1997	22.5
1998	22.1
1999	22.2
2000	21.2
2001	20.8
2002	20.3
2003	19.9
2004	19.3
2005	18.8
2006	18.2
2007	17.6
2008	17.2
2009	16.5
2010	15.9
2011	15.4
2012	14.9
2013	14.5
2014	14.1
2015	13.8
2016	13.5
2017	13.2
2018	13
2019	12.8
2020	12.6
2021	12.4
2022	12.4