Period	Percent
1970-01-01	6.55
1975-01-01	10.10
1980-01-01	15.30
1985-01-01	23.09
1990-01-01	34.02
1995-01-01	39.98
2000-01-01	51.13
2005-01-01	55.49
2010-01-01	62.02