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What is the Meaning of an IQ Test Score?

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The Bell Curve

The Bell Curve

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Your child takes an IQ test. You get the score back. You learn that your child, with an IQ score of 150, falls into the highly gifted range. What does that mean?

Before you can understand what it means for a child to be highly gifted (or moderately gifted or profoundly gifted), you need to understand what IQ scores represent. An IQ score is an Intelligence Quotient, which is a measure of intelligence, primarily reasoning ability. The higher the score, the greater the reasoning ability.

If we took everyone's IQ scores and plotted them, we would see they they would be distributed in a normal bell curve. That means that most scores would fall somewhere in the center of that bell curve. The score in the absolute center of the bell curve is 100 and that is where we would expect most scores to fall, or where we expect them to cluster.

As the scores move away from the norm (100), we will find fewer and fewer scores. However, to make the numbers meaningful, we need to be able to measure the variability of the scores. That is the purpose of standard deviations, which is, quite simply, the average distance scores are from the norm. Statisticians determine theStandard deviation of data through a specific formula.

Categories of Giftedness

Once you understand these scores and how they fit in a bell curve, you can better understand the different categories of giftedness. Why is a score between 115 and 130 considered mildly gifted? Why is a score of 131 and 145 highly gifted? The answer lies in the standard deviation of the scatter of IQ scores on the bell curve. The standard deviation used in many tests, including the Weschsler IQ test, is 15. The majority of test scores (about 70%) fall somewhere between one standard deviation below and one standard deviation above 100. That means most scores are somewhere between 85 and 115. Those scores are considered the "average" or normal intelligence range.

The farther the score is from 100, the fewer people we will find with that score. If we move one additional standard deviation below and one additional standard deviation above 100, we will find about 25% of the scores falling within those ranges. In other words, people with IQs between 70 and 85 and between 115 and 130 make up about 25% of the population. That leaves only about 5% of the population who will have scores somewhere beyond those first two standard deviations away from the norm.

What does this score variance mean for the gifted child?

People often want to lump all gifted children into one group, assuming that all gifted children have the same needs. Nothing could be farther from the truth. A good way to understand the difference in the needs of these different groups of gifted children is to consider how far they are from the norm of 100:
  • Mildly Gifted -- 115 to 129
  • Moderately Gifted -- 130 to 144
  • Highly Gifted -- 145 to 159
  • Exceptionally Gifted -- 160 to 179
  • Profoundly Gifted -- 180
If you look at the scores for each group, you will notice that each category represents one standard deviation from the norm. To understand the difference one standard deviation can make, consider the scores below 100. One standard deviation on either side of 100 is within the normal, or average range. Move down one more standard deviation and you move into the range of moderate intellectual functioning (70-84). Children with scores in this range qualify for special academic services. Moving down another standard deviation takes us into the range of moderately retarded (55-70). The farther a child's score is from the norm, the more he or she needs special academic services.

Now move in the opposite direction from 100. An IQ score up to one standard deviation above 100 is considered normal, or average. Move up one standard deviation is mildly gifted. That means that a child with a score of 130 is as different from a child with an IQ of 100 as is the child with an IQ of 70, a score which definitely qualifies a child for special services. Move up one more standard deviation and we move into the range of moderately gifted (130-144). The same range on the other side of 100 is the mildly retarded range.

No educator would believe that every child with an IQ anywhere below 70 need the same academic services that every other child in the range would need. The standard deviations below 100 are meaningful. They are no less meaningful when they are above 100.

Cautions about IQ Scores
IQ testing is not a science. It may seem that way at times, but it's not. Scores from tests are really estimates based on test performance on a particular day. There is always a margin of error. The "actual" score could be higher or it could be a little lower (within the margin of error). However, it is also important to note that the score won't change substantially. That is, a child who gets a score of 140 did not get that score because he or she had a "good day." Some people may tell parents that about their children, but that's not true. The highest score a child gets will be the best reflection of the child's IQ (within the margin of error). An average child cannot get a score that high just because she ate a good breakfast and felt good that day!
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