Year	Percent change (Units)
1980	30.200
1981	25.700
1982	28.900
1983	27.100
1984	36.100
1985	33.300
1986	32.400
1987	29.900
1988	31.200
1989	25.800
1990	36.400
1991	25.200
1992	20.700
1993	26.100
1994	37.900
1995	26.770
1996	21.049
1997	16.138
1998	12.778
1999	7.889
2000	5.956
2001	5.147
2002	4.555
2003	4.429
2004	4.138
2005	4.359
2006	7.251
2007	7.028
2008	10.276
2009	12.144
2010	7.192
2011	12.691
2012	16.001
2013	7.870
2014	6.132
2015	5.588
2016	5.170
2017	5.319
2018	3.509
2019	3.448
2020	3.289
2021	3.690
2022	4.350
2023	4.036
2024	4.038
2025	4.026
2026	4.012
2027	4.007
2028	4.030
2029	3.968