Comparatio, which is simply Latin for "comparison," is a rhetorical strategy that uses a comparison to persuade an audience. It is a form of figurative language, often a metaphor or simile, that draws on the audience's previous knowledge and experience. Most often, this rhetorical device is used to elicit a particular emotional reaction, but it can also be used to aid logical or factual comprehension of a subject. Comparatio should not be confused with compa ratio or compa-ratio, which is a method of determining a person's salary.
Effective comparatio has emotional content that is appropriate to the speaker or writer's aims. For instance, a speaker might want to persuade his or her audience that a particular ideology that is growing in popularity is having a detrimental effect. This speaker would do better to compare the ideology to cancer cells, rather than to bunny rabbits. Both have a reputation for multiplying and spreading quickly, but of course cancer has negative connotations, while rabbits are likely to be neutral or even positive.
![](http://images.languagehumanities.org/slideshow-mobile-small/brit-politician.jpg)
In addition to containing appropriate emotional content, comparatio should make a comparison between items that are logically comparable. As an example, a writer might be trying to encourage a free flow of ideas that is not hampered by shyness or fear. The writer might compare a poor attitude to a flat tire on a bike, but a better analogy might be to say that a person who will not express his or her ideas is like a jar with the lid screwed on too tight. Both analogies have the appropriate emotional content — neither a flat tire nor a jar that will not open is commonly viewed as a positive — but a lid that is stuck stops something from coming out, as shyness keeps a person's ideas from coming out. A flat tire also makes movement more difficult, but in a less logically comparable way.
Using an analogy to convey factual information can also be considered a form of comparatio. A doctor might need to communicate with a patient by means of analogy, for instance, if the patient lacks the technical knowledge to easily understand the function of a procedure he or she needs. Unlike other forms of comparatio, this would not necessarily have persuasion as its ultimate goal.