Period	Percent
1970-01-01	0.08
1975-01-01	0.07
1980-01-01	0.10
1985-01-01	0.13
1990-01-01	0.22
1995-01-01	0.38
2000-01-01	0.33
2005-01-01	0.42
2010-01-01	0.50