The 'comparison' group
These nouns refer to ways of comparing one thing with another, referring to something, or using something as the basis of an idea or statement.
They were sun-tanned and healthy-looking and the women were elegantly slim. By comparison he felt physically and mentally washed out.
All these were the necessary and outward signs of progress and, by extension, America's preeminence in the world.
The United States has managed to secure UN endorsement for the steps it has taken, including by implication the military build-up.
Reference is followed by a prepositional phrase beginning with to.
by analogy by comparison by contrast by extension by implication by inference by reference to ...
The action may then be explained by reference to the state of mind that produced it.