Year	Percent (Units)
1980	6.378
1981	6.252
1982	6.179
1983	6.127
1984	6.034
1985	5.953
1986	5.893
1987	5.761
1988	5.717
1989	5.731
1990	5.824
1991	5.970
1992	5.477
1993	5.320
1994	5.291
1995	5.182
1996	5.030
1997	4.920
1998	4.884
1999	4.809
2000	4.722
2001	4.687
2002	4.551
2003	4.342
2004	4.173
2005	4.015
2006	3.961
2007	3.873
2008	3.799
2009	3.599
2010	3.562
2011	3.563
2012	3.455
2013	3.426
2014	3.465
2015	3.468
2016	3.575
2017	3.596
2018	3.506
2019	3.448
2020	3.415
2021	3.311
2022	3.261
2023	3.154
2024	3.063
2025	3.007
2026	2.958
2027	2.901
2028	2.838
2029	2.772