Year	% of revenue
1972	29.96389891696751
1973	24.90909090909091
1974	31.353135313531354
1975	31.52676801057502
1976	29.942693409742123
1977	29.046563192904657
1978	26.481240040562078
1979	27.854900016526194
1980	23.36029626742033
1981	24.604430379746837
1982	26.185085092396466
1983	23.96085578051277
1984	24.954854088413754
1985	25.611579420478627
1986	27.321115537848605
1987	26.270984429679974
1988	26.74880584349578
1989	28.73263888888889
1990	26.957192441187814
1991	29.801670146137788
1992	30.203924586379376
1993	29.566833774573308
1994	29.507574319450246
1995	31.284585565057853
1996	30.688893795539858
1997	29.016923051170888
1998	28.714713946781455
1999	26.647765380159743
2000	28.920417864898173
2001	25.796384677971524
2002	22.549897420766765
2003	19.349734010225806
2004	18.735544887679158
2005	20.473489700777016
2006	20.575463091092583
2007	20.78511631608037
2008	22.002068468933654
2009	29.675423191513005
2010	29.69432927879542
2011	30.369736771191658
2012	32.23047950891228
2013	32.39061411545014
2014	33.95849767609789
2015	34.01821344058976
2016	29.123023698201557
2017	33.17429562522911
2018	31.457685653964866
2019	32.3886432482764
2020	31.81938924601924