Period	Percent
1970-01-01	0.12
1975-01-01	0.20
1980-01-01	0.28
1985-01-01	0.37
1990-01-01	0.39
1995-01-01	0.44
2000-01-01	0.49
2005-01-01	0.42
2010-01-01	0.36