Year	% of revenue
1972	13.461538461538462
1973	17.307692307692307
1974	13.432835820895523
1975	22.47191011235955
1976	21.929824561403507
1977	24.528301886792452
1978	27.376425855513308
1979	28.31215970961887
1980	27.085377821393525
1981	28.693368351936964
1982	23.903420523138834
1983	21.722846441947567
1984	19.468285534854513
1985	22.057719128848834
1986	20.82273018963953
1987	21.540407081777673
1988	14.392059553349876
1989	18.322442992398987
1990	16.51167566298991
1991	9.545038003342137
1992	9.481153343912679
1993	11.480977165006903
1994	10.201922132225464
1995	9.91579468763544
1996	8.904246195576297
1997	8.030595374106456
1998	8.834552161954297
1999	8.727828241123039
2000	8.103762191720358
2001	7.897041782571493
2002	6.7045740328407835
2003	6.2602601480152495
2004	6.075179321126068
2005	5.687255890949536
2006	4.0872534953101844
2007	2.3748409910100974
2008	2.5507393285675235
2009	2.148766009223466
2010	2.121274146682399
2011	1.392711217055906
2012	1.3901424146890904
2013	1.450884307630373
2014	1.4344226183996305
2015	1.4721033506461803
2016	1.339914468998404
2017	1.1566062345729908
2018	1.029205551949534
2019	0.9329301742835545
2020	0.8833655975803928
2021	0.7859058523521069