Missing words usually appear in papers that have not been proofread carefully:
Six percent of the people used to help control diarrhea .
Out of context, the problems with this sentence are obvious. But when proofreading your own work, picking up such errors can be a bit difficult. When most people read, they don't study each and every word. Instead, they barely glance at some words because they have already predicted what will come next. Frank Smith explains:
Everyone predicts—including children—all the time. . . . And all our expectations, our predictions, can be derived from only one source, our theory of the world.
We are generally unaware of our constant state of anticipation for the simple reason once again that our theory of the world works so well. Our theory is so efficient that when our predictions fail, we are surprised. We do not go through life predicting that anything might happen—indeed, that would be contrary to prediction, and in that case nothing could surprise us. The fact that something always could rhinoceros takes us by surprise—like the word rhinoceros a few words ago—is evidence that indeed we always predict and that our predictions are usually accurate. (Understanding Reading)
When you reread something you have read several times, something with which you are very familiar, you are particularly likely to predict what is coming next. And when you are predicting (rather than studying individual words), you will sometimes overlook missed words and other typos.
The student who wrote the sample sentence knew what he was trying to say, so he never noticed that he did not include all of the information an outside reader would need:
Six percent of the people in one study used medical marijuana to help control diarrhea.
When you are proofreading your own work, make sure you read slowly. Try to read your text the way someone who has never read it before would read. Some writers read their papers backwards (last sentence, next-to-last sentence, etc.) because they find they are more likely to find errors when they read their sentences out of context. See pages 21-32 of The Longman Concise Companion for additional hints on proofreading your own work.