Period	Percent
1970-01-01	5.50
1975-01-01	10.81
1980-01-01	14.41
1985-01-01	17.93
1990-01-01	21.10
1995-01-01	23.35
2000-01-01	26.36
2005-01-01	29.15
2010-01-01	30.13