Dr. Hilde L. Mosse explains why having children read silently before they can read accurately is a bad idea:
Silent reading is a higher performance than reading aloud; it is much more difficult to do.
Children call silent reading “reading in my mind.” Like many other formulations invented by children, this one expresses a complicated process in a simple way, exactly as the child experiences it, and comes right to the central point. Silent reading is indeed based on images that exist only in the mind. It is mnemic reading, which means that it is based on the formation of memory images of how words sound when they are spoken. As I discussed in the section on the formation of conditioned reflexes, engram complexes between the groups of letters forming the word and the sound of the word must have been firmly established before silent reading becomes possible. Without this foundation children substitute guessing for reading, and this disturbing habit may last throughout their lives. A consequence of this lack of a firm foundation is that many children never feel entirely sure of their reading or feel sure when they should not. They may read words wrongly or read the wrong words for years and may completely distort and misunderstand the meaning of these words, unless their reading is constantly checked by oral reading and also by writing spontaneously and to dictation. Because this is not stressed sufficiently in current curriculum practice, severe reading disorders are obscured and their diagnosis is delayed. Children can acquire an astonishing facility with guessing and with filling in the right word or marking the right answers on silent reading tests. They can also pick up the content by ear when they are supposed to give a report on a text they were supposed to have read silently by themselves. Silent reading should therefore be introduced much later than is the current curriculum practice. A child should not be permitted to read silently before he and the teacher are sure that he can pronounce every word he is supposed to read silently, and can read it orally.
Twelve-year-old Darryl understood the importance of this when he told me, “If you read in your mind, you might say the wrong word.” This was his explanation of why he still liked to read his homework out loud to his mother. These were his own words, and he chose them carefully. He deliberately said “the wrong word” and not “the word wrong.” He was not worried about mispronouncing a word, but wanted to be sure that he was reading the right word, so that he cold understand the text correctly. He had fortunately learned that the meaning of a word depends on its sound and not its configuration. He had overcome his organic reading disorder, which I had diagnosed when he was 7 years old.
The difference between the silent and the oral reader is that the oral reader actually hears his own pronunciation of the word, whereas the silent reader hears only the memory image and sensation of it (Schmitt, 1966, p. 85.). Silent reading is based on “inner” or “mental” hearing and speech. The silent reader whispers or starts to read aloud only when he tries to figure out an unfamiliar word. The silent articulation of words, which in the beginning accompanies silent reading, can be dropped only after intensive practice, so that in the end the child reads faster than he speak. This is, of course, the ultimate goal but it cannot be reached until the technical basis of the reading process has become completely automatic, so that the child no longer has to focus his attention on it. Only then is he free to concentrate fully on the content of the text, and only then can the highest cortical structures be activated so that understanding and the forming of association can take place at the highest level, and so that even creative thinking become possible while reading. Perfect silent reading is done without lip and tongue movement. It is really silent and much faster than oral reading.
Mosse, Hilde L., The Complete Handbook of Children's Reading Disorders: You Can Prevent or Correct Learning Disorders. Beaverton, OR: Riggs Institute Press, 1982, pp. 112-114
For more on this topic, see the Reading Rockets article, For Students Who Are Not Yet Fluent, Silent Reading Is Not the Best Use of Classroom Time, by: Jan Hasbrouck