Year	Percent (Units)
1980	4.310
1981	4.266
1982	4.356
1983	4.306
1984	4.186
1985	4.107
1986	4.063
1987	4.015
1988	4.023
1989	4.049
1990	4.006
1991	3.951
1992	3.613
1993	3.522
1994	3.501
1995	3.449
1996	3.367
1997	3.312
1998	3.338
1999	3.336
2000	3.308
2001	3.293
2002	3.240
2003	3.139
2004	3.066
2005	2.978
2006	2.899
2007	2.819
2008	2.746
2009	2.679
2010	2.595
2011	2.553
2012	2.451
2013	2.463
2014	2.423
2015	2.424
2016	2.458
2017	2.443
2018	2.403
2019	2.381
2020	2.267
2021	2.266
2022	2.247
2023	2.199
2024	2.148
2025	2.111
2026	2.079
2027	2.046
2028	2.014
2029	1.981