Year	Percent change (Units)
1981	26.290
1982	23.404
1983	34.483
1984	27.564
1985	39.196
1986	28.520
1987	32.303
1988	30.786
1989	36.851
1990	18.624
1991	37.760
1992	20.736
1993	26.108
1994	37.972
1995	26.771
1996	15.546
1997	15.353
1998	11.260
1999	6.962
2000	5.529
2001	4.854
2002	4.404
2003	4.563
2004	4.102
2005	4.982
2006	6.659
2007	6.423
2008	13.474
2009	12.226
2010	5.553
2011	19.754
2012	12.062
2013	5.561
2014	4.752
2015	6.828
2016	5.049
2017	3.961
2018	3.250
2019	3.850
2020	3.154
2021	4.160
2022	4.851
2023	3.968
2024	4.007
2025	3.991
2026	3.979
2027	3.971
2028	3.968
2029	3.968