cranial index vs ratio

cranial index and ratio both are nouns.

Word	Noun	Adjective	Verb	Adverb
cranial index	Yes	No	No	No
ratio	Yes	No	No	No

As nouns, ratio is a hypernym of cranial index; that is, ratio is a word with a broader meaning than cranial index:

cranial index: ratio (in percent) of the maximum breadth to the maximum length of a skull
ratio: the relative magnitudes of two quantities (usually expressed as a quotient)

cranial index (noun)	ratio (noun)
ratio (in percent) of the maximum breadth to the maximum length of a skull	the relation between things (or parts of things) with respect to their comparative quantity, magnitude, or degree
	the relative magnitudes of two quantities (usually expressed as a quotient)