Franzia decides that she is an excellent student because her grades are much higher than her younger siblings. her judgment is based on a comparison.

The answer is "Downward" comparison. 

People tend to compare themselves in two ways, upward comparison and downward comparison. Both of these approaches have their pros and cons. Upward comparison mean when someone compares himself with those who are higher than him and opposite is the definition of downward comparison where the people to whom the comparison is made are lesser than the person making the comparison.