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# Where Does Intelligence Come From? (page 3)

By John Wiley & Sons, Inc.
Updated on Jan 1, 2011

#### Here Come the Numbers: Put on Your Safety Belts—We Are in for a Numbers Ride!

In this section, we will talk about how scores from IQ tests can help us understand a child's ability in terms of strengths and weaknesses, and help us form possible expectations for performance. Standard scores are a common measurement that allow us to compare scores across a number of different types of assessment instruments.

#### Standard Scores

IQ tests report their scores as standard scores. In our initial example, we discussed how measuring a child's temperature provides a comparison with the normal standard score, which for temperature is 98.6 degrees on the Fahrenheit scale. So regardless of which thermometer you use, the numbers will represent the same scale. Similarly, scores on different tests (for example, achievement tests and IQ tests) can be compared if the scores are all using the same scale. Standard scores are based on the normal distribution of scores that look like the bell-shaped curve presented in Figure 7.1. In this distribution, the majority of scores fall in the middle or the average range. The average score (called the mean) is 100, which falls in the exact middle of the bell. If we were to test 2,000 people, we would find that that 50 percent of the people would score above 100 and 50 percent would score below 100. However, we would also find that the majority of people who took the IQ test had scores that deviated from the mean (called the standard deviation) by 15 points on either side. Roughly 68 percent of the population would score within one standard deviation of the mean:

• 34 percent would obtain an IQ score between 85 and 100 (100 – 15);
• 34 percent would obtain an IQ score between 100 and 115 (100 + 15).

What about 2 standard deviations above or below the mean? How many would score in that area? Well, if we look again at our bell shape, we see that the biggest concentration of population is in the center within one standard deviation on either side. However, as we move toward the ends of the bell on either side, we are losing more and more people. If we looked at these narrow portions under the bell, representing 2 standard deviations (15 + 15) or 30 points from the middle on either side, we would find about 2 percent of the population would score within two standard deviations of the mean:

• 2 percent would obtain an IQ score at or below an IQ of 70
• 2 percent would obtain an IQ score at or above an IQ of 130.