Sometimes writers get caught up in the moment and cannot resist a good digression. Even though you might enjoy such detours when you chat with friends, unplanned digressions usually harm a piece of writing.

Mariah stayed close to her outline when she drafted the three body paragraphs of her essay she tentatively titled “Digital Technology: The Newest and the Best at What Price?” But a recent shopping trip for an HDTV upset her enough that she digressed from the main topic of her third paragraph and included comments about the sales staff at the electronics store she visited. When she revised her essay, she deleted the off-topic sentences that affected the unity of the paragraph.

Read the following paragraph twice, the first time without Mariah’s changes, and the second time with them.

Nothing is more confusing to me than choosing among televisions. It confuses lots of people who want a new high-definition digital television (HDTV) with a large screen to watch sports and DVDs on. You could listen to the guys in the electronics store, but word has it they know little more than you do. They want to sell what they have in stock, not what best fits your needs. You face decisions you never had to make with the old, bulky picture-tube televisions. Screen resolution means the number of horizontal scan lines the screen can show. This resolution is often 1080p, or full HD, or 768p. The trouble is that if you have a smaller screen, 32 inches or 37 inches diagonal, you won’t be able to tell the difference with the naked eye. The 1080p televisions cost more, though, so those are what the salespeople want you to buy. They get bigger commissions. The other important decision you face as you walk around the sales floor is whether to get a plasma screen or an LCD screen. Now here the salespeople may finally give you decent info. Plasma flat-panel television screens can be much larger in diameter than their LCD rivals. Plasma screens show truer blacks and can be viewed at a wider angle than current LCD screens. But be careful and tell the salesperson you have budget constraints. Large flat-panel plasma screens are much more expensive than flat-screen LCD models. Don’t let someone make you by more television than you need!