Unity in writing means that all the points you make are related to your main point; they are unified in support of it. As you draft a paragraph or an essay, you may detour from your main point without even being aware of it, as the writer of the following paragraph did with the underlined sentences. (The main point in the paragraph has been double-underlined.)

If you want to drive like an elderly person, use a cell phone while driving. A group of researchers from the University of Utah tested the reaction times of two groups of people — those between the ages of sixty-five to seventy-four and those who were eighteen to twenty-five — in a variety of driving tasks. All tasks were done with hands-free cell phones. That part of the study surprised me because I thought the main problem was using only one hand to drive. I hardly ever drive with two hands, even when I’m not talking to anyone. Among other results, braking time for both groups slowed by 18 percent. A related result is that the number of rear-end collisions doubled. The study determined that the younger drivers were paying as much — or more — attention to their phone conversations as they were to what was going on around them on the road. The elderly drivers also experienced longer reaction times and more accidents, pushing most of them into the category of dangerous driver. This study makes a good case for turning off the phone when you buckle up.

Detours such as the underlined sentences in the example weaken your writing because readers’ focus is shifted from your main point. As you revise, check to make sure your paragraph or essay has unity.