DECIDING TO WRITE AN EVALUATION. It’s one thing to offer an opinion, yet it’s an entirely different matter to back up a claim with reasons and evidence. Only when you do will readers (or listeners) take you seriously. But you’ll also have to convince them that you know how to evaluate a book, a social policy, a cultural trend, or even a cup of coffee by reasonable criteria. It helps if you can use objective standards to make judgments, counting or measuring varying degrees of excellence. But perhaps more often, evaluations involve people debating matters of taste — an act that draws good sense and wit into the mix. Here’s how to frame this kind of argument (for more on choosing a genre, see the Introduction).

Explain your mission. Just what do you intend to evaluate and for whom? Maybe you’ll assess or rank particular products, productions, or performances or challenge opinions others have offered about them. Or maybe you want to turn social critic, making people aware of their failures or foibles. Or perhaps you see yourself as a sports pundit or fashion guru. You need to share your intentions and credentials with readers.

Establish and defend criteria. Criteria are the standards by which objects are measured: A good furnace should heat a home quickly and efficiently. Successful presidents leave office with the country in better shape than when they entered. When readers are likely to share your criteria, you need to explain little about them. But when readers might object, prepare to defend your principles. (develop ideas) And sometimes you’ll break new ground — as happened when critics first asked, What is good Web design? or Which are the most significant social networks? In such cases, criteria of evaluation had to be invented, explained, and defended.

Offer convincing evidence. Evidence makes the connection between an opinion and the criteria of evaluation that support it. It comes in many forms: facts, statistics, testimony, photographs, and even good sense and keen observations. If good furnaces heat homes quickly and efficiently, then you’d have to supply data to show that a product you judged faulty didn’t meet those minimal standards. (It might be noisy and unreliable to boot.)

Offer worthwhile advice. Some evaluations are just for fun: Consider all the hoopla that arguments about sports rankings generate. But done right, most evaluations and reviews provide usable information, beneficial criticism, or even ranked choices — think restaurant or entertainment reviews on Yelp.