Year	Percent (Units)
1980	4.581
1981	4.431
1982	4.432
1983	4.378
1984	4.328
1985	4.295
1986	4.271
1987	4.246
1988	4.232
1989	4.221
1990	4.140
1991	4.100
1992	3.721
1993	3.619
1994	3.590
1995	3.563
1996	3.472
1997	3.398
1998	3.365
1999	3.305
2000	3.274
2001	3.258
2002	3.177
2003	3.058
2004	2.945
2005	2.836
2006	2.744
2007	2.644
2008	2.544
2009	2.420
2010	2.339
2011	2.267
2012	2.152
2013	2.065
2014	2.002
2015	1.998
2016	2.078
2017	2.062
2018	2.009
2019	1.964
2020	1.841
2021	1.875
2022	1.886
2023	1.846
2024	1.803
2025	1.759
2026	1.709
2027	1.662
2028	1.625
2029	1.590