Period	Percent
1970-01-01	4.50
1975-01-01	6.85
1980-01-01	10.10
1985-01-01	11.66
1990-01-01	15.38
1995-01-01	20.69
2000-01-01	25.53
2005-01-01	32.74
2010-01-01	20.87