Often comparisons go beyond a simple detailing of likenesses and differences; we usually use comparisons in order to evaluate which is the better choice of the two possibilities. In order to do so, we must determine which attributes of the alternatives in question are relevant to our evaluation. For instance, if we are comparing two different Italian restaurants, we wouldn't concentrate on their similarities, because this wouldn't help us choose between them. But we may want to consider other points: the location, the price of entrees, the quality of the food, how fast the service is, and even which wines they stock.

Now, after determining which categories are relevant to consider, we must decide how to structure the formal comparison of the two alternatives. Two basic patterns are used, the "block" and the "alternating." In the introduction, both patterns mention the similarities we have found between the two restaurants, but also included is the final recommendation. The block pattern lays out all the categories we've considered for Restaurant A first and then Restaurant B, being careful to discuss them in the same order. The conclusion reasserts the final recommendation, and thus ties in to the introduction, creating a unified essay. The alternating pattern, on the other hand, discusses the relevant attributes in pairs, usually disposing of the second choice first and then finishing with the winner, for as many categories as we've thought relevant. The conclusion works the same as for the block pattern, tying the paper together through its reassertion of the recommendation in the introduction.

When is it better to use the block or alternating patterns, and how should the categories be ordered? Most writers use the alternating mode when the comparison has many different points and/or when the paper is longer. The block mode works well with only a few points of comparison and for shorter papers. Of course, the writer will also need to consider the question of how readily the audience can follow the comparison through these patterns. A common strategy for ordering the categories is to try to take up closely related points sequentially-this will be easier on your reader. Also, the most significant point is often held in reserve so that the writer can conclude with a punch that sticks with the reader.

Finally, the analogy is a special type of comparison most often used to explain an unfamiliar item by comparing it with a familiar one. These two essentially different items that have some hidden similarities. Keep in mind these tips: (1) the reader must be familiar with at least one of the items, (2) the two items must actually have significant similarities, (3) obvious comparisons should be avoided, and (4) the analogy shouldn't be over-extended to the point of turning off your reader.