Intellectual assessments often measure verbal reasoning, nonverbal reasoning, working memory and processing speed, as with the Wechsler Intelligence Scale for Children: Fourth Edition (WISC-IV). The Full Scale IQ (FSIQ), a combination of all the subtests, is considered to be the best predictor of overall cognitive functioning. However, when the working memory and/or processing speed are either significantly larger or smaller than the verbal and/or nonverbal areas, the General Ability Index (GAI) is utilized as a more accurate representative of intellectual ability. The GAI can be used the same way as the FSIQ to determine learning disabilities when there is a severe discrepancy between ability and achievement.

Obtaining Standard Scores for Intelligence and Achievement

Step 1

Calculate the standard scores.

Score and calculate your intelligence and achievement protocols for the student undergoing the psycho-educational evaluation. Make sure you have all the standard scores for the areas of achievement (reading, math, writing, etc.). These will be needed when determining a learning disability.

Step 2

Add the sums to find the GAI.

Obtain the GAI sum of scaled scores. To find the GAI, add together the sum of scaled scores for the verbal area and the sum of scaled scores for the nonverbal area, located on the intellectual assessment protocol. For example, if the sum of scores for the verbal is 30 and the nonverbal is 36, then the GAI sum of scaled scores is 66.

Step 3

Find the GAI.

Look up the GAI sum of scaled scores to find the standard score. This score is comparable to the FSIQ. To find the GAI score, you will have to look up the sum of scaled scores equivalent in a chart. See the Resource section of this article for the GAI chart when using the WISC-IV. Using the previous example (sum of scores 66), the GAI is 106. This is the score you will now use instead of the FSIQ.

Determine Discrepancies Between Ability and Achievement

Step 1

Find the predicted score.

Find the predicted level of achievement performance. Based on the student's GAI, this is the score he should be obtaining. The formula for finding the predicted level is Y=rxy (X-100) + 100. Y is the predicted achievement score. The rxy represents the correlation between the ability and achievement assessments. X is the GAI. For example, using the WISC-IV and WIAT-III Achievement, the correlation is .84. Remember, the GAI score is 106. Y=.84 (106-100) + 100=105.04. Round the score to a whole number (Y=105).

Step 2

Determine the point-difference criteria.

Determine the point-difference criteria. This is the number of points the standard score needs to differ from the predicted level to be considered discrepant. Because most intellectual assessments have a standard deviation of 15, that number is most often used as the criterion value. Therefore, using the point-difference of 15 and the example of Y=105, any score of 90 or below constitutes a severe discrepancy between ability and achievement.

Step 3

Find discrepant scores.

Compare the standard scores from the achievement test with the discrepancy number (in this case, 90 and below). Any score that falls within the discrepant range can be classified as an area of learning disability. For example, a score of 85 on reading comprehension can mean that a student is displaying a learning disability in reading comprehension.