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.
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.