David J Carey
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Wechsler Intelligence Scale

Wechsler Intelligence Scale for Children


The most common assessment instrument used by psychologists is the Wechsler Intelligence Scale for Children and will therefore be the one we look at. In using this test as an example, you will hopefully get a reasonable picture of how an assessment is carried out. The Wechsler test is, essentially, a test of intelligence. It has been in use for over fifty years and has been revised numerous times to keep it up to date.

The test is divided into two sections with each section containing a number of subtests. The two broad sections of the test are the

• Verbal Scale

• Performance Scale

Successful completion of any item on any of the Verbal subtests requires a verbal response. On the Performance subtests, the person must do something in response to a question or task. When the entire test has been administered, the assessor calculates what is called a Composite Score, a score that takes into account both sections. Because it is a test of intelligence, the test scores obtained are called IQ scores and you will see the results stated in this format:

• Verbal Scale IQ

• Performance Scale IQ

• Full Scale IQ (the composite score)


The Full Scale score, according to the standard interpretation, indicates the level of a person's intelligence. A Full Scale score in the range of 90 to 110 is considered average; the person can be said to have average intelligence.

In addition to looking at the Full Scale score, the three scores, (verbal, performance and full scale) can be compared against one another. What is expected in most people is that the three scores will cluster close enough  together to indicate that the individual's verbal and performance skills are evenly developed. When there is a large difference between the two subtest scores (verbal and performance), it may indicate learning problems. 

This is as good a time as any to introduce the reader to some of the common terms used in assessment:

• Percentile scores

• Reading age scores

• Standard scores

Children are frequently referred for assessment after reading or maths tests have been administered to the entire class. The most common whole-class tests in use are the Drumcondra tests (reading) and the Micra T test (mathematics). A child's results in these tests are reported in what are called percentile scores. A percentile score indicates where a child stands in comparison to a sample of children in his or her own age, on a given task. A score at the 50th percentile on the Drumcondra test means that the child is well within the middle range (49 children out of 100 score higher, 49 score lower).

Some tests yield what are called reading age scores. Reading age scores do not yield significant information, their use has been criticised and has been discouraged in the learning support teachers' written guidelines. A two-year difference in reading age in 5th class may not be terribly significant.

Standard scores are also frequently reported following assessment. The average standard score is 100, which is at the 50th percentile, meaning the child's score isn't significantly different in that test than other, same- age children. Standard scores must differ from one another by about fifteen points in order for the difference to be of any real significance. About two-thirds of all children have standard scores on a test that are between 85 and 115, that is, the 16th percentile and the 84th percentile (see table below). Scores in this range are not particularly noteworthy (there are exceptions to this, which will be presented when we explain tests of children's intelligence).

The following chart will be helpful in translating standard scores, scale scores, standard deviations, and percentile scores into understandable and meaningful information. Standard deviations tell us how much confidence we can place in a given score. Any time a test is administered there will be a certain range of scores obtained that don't have any significance in the actual test results. In psychological assessment the usual standard deviation of significance is three points or more. So if a child measures 12 points on a test and 11 on another there is no real significance to this difference. Although an oversimplification it is helpful to consider the standard deviation in scores to determine whether or not a strength or weakness is actually present upon assessment.

 The most useful scores to interpret for common sense purposes are therefore percentile scores. I recommend you ask for percentile scores when test results are being reported. Most importantly, do not expect reading- or mathematics-age scores to be useful for educational planning or for reviewing the effectiveness of educational interventions. We will refer to percentiles again throughout this section.


Standard Scores 

The Verbal Scale, Performance Scale, and Full Scale scores are all Standard Scores. Previously I stated that standard scores all have 100 as their average, with the range of average being from 90 to 110. About two-thirds of all children will score between 85 and 115 on these three scales and scores within this range are not highly significant.

At the risk of getting bogged down in too much information, it's worth having a more detailed look. For example, let's take a look at the Verbal Scale. The subtests that are administered are in bold and I have included what they are trying to assess:

• Information: factual knowledge, long-term memory, recall.

• Similarities: abstract reasoning, verbal categories and concepts.

• Arithmetic: attention and concentration, numerical reasoning.

• Vocabulary: language development, word knowledge, verbal fluency.

• Comprehension: social and practical judgment, common sense.

• Digit Span: short-term auditory memory, concentration.


On the Performance Scale, the following subtests are administered (bold) and what they are trying to assess is indicated:

• Picture Completion: alertness to detail, visual discrimination.

• Coding: visual-motor coordination, speed, and concentration.

• Picture Arrangement: planning, logical thinking, social knowledge.

• Block Design: spatial analysis, abstract visual problem solving.

• Object Assembly: visual analysis and construction of objects.

• Symbol Search: visual-motor quickness, concentration, persistence.

• Mazes: fine motor coordination, planning, following directions.


An example will help illustrate the fine points of interpreting this test. Suppose Patricia is referred for an educational psychological assessment, having progressed through Stages One and Two.


The Wechsler test is administered and she obtains the following results (this is a crude example for illustrative purposes and the numbers are not meant to be accurate representations of what a real test profile would look like). Individual subtest scores range from a low of one to a high of nineteen. Remember that differences of three points or less between them are not particularly significant. When the difference exceeds three points it may indicate a difficulty with the underlying brain processing tasks that were described above.


Verbal Scale                Performance Scale 

Information 8              Picture Completion 9

Similarities 3               Coding 10

Arithmetic 9                Picture Arrangement 11

Vocabulary 9               Block Design 2

Comprehension 18      Object Assembly 9

Digit Span 9                Symbol Search 8

Mazes 14 


Using the conversion tables available in the Wechsler test manual, the results of these subtests yield the following scale scores: 

Verbal Scale IQ 109

Performance Scale IQ 113

Full Scale IQ 110


Patricia is in the average range, right? Looking at the three Scale scores, you would think so. But if we take a closer look at the individual subtest scores, something interesting comes into view. On two subtests that assess abstract thinking (Similarities and Block Design), Patricia's subtest scores are quite low. Subtest scores have an average of ten and there is little significance in a variation of three. However, Patricia's score of 2 on Block Design and 3 on Similarities indicates a real weakness in abstract thinking, verbally and non-verbally, despite her average intelligence. This weakness may well indicate learning problems.

I described percentile scores earlier. These scores help us to compare a child's test results with those of other, same-age children. Let's see how Patricia compares with other girls her age by looking at the percentile scores that correspond to each of her scores above, as follows:

Verbal Scale        Performance Scale       
Scale Score     percentile          Scale   percentile    
Information 8 25 Picture Compilation 9 37
Similarities 3 1 Coding 10           50
Arithmetic 9 27 Picture Arrangement 11   63
Vocabulary 9 27 Block Design 2       1
Comprehension 18 99 Object Assembly 9   37
Digit Span 9 37 Symbol Search 8    25
    Mazes 14    91


The results of all these subtests yield the following scale scores:

Scale Score               percentile
Verbal Scale IQ 109        73
Performance Scale IQ 113   81
Full Scale IQ 110   75


Taking a look at the percentile scores tells us more about how Patricia compares to children her own age. 

Now, let's suppose that Patricia was initially referred because she was having considerable difficulty learning to read. I was at pains to point out in the earlier section that the assessor must take into account all the factors that might result in Patricia's difficulty, before drawing conclusions She may have had health problems which caused her to miss one-third of the school year over each of the past several years; what if her parents were members of the Travelling Community and moved her from school to school five times each year? What if, for the past two years she has had three different teachers, as a result of staff illness, and two of them had no teaching qualification? There may be personal issues (family bereavement etc) that may have relevance. Any of these factors, and more, could be the real cause of Patricia's reading problems. The assessor will have to take everything into account and put it together in a way that makes sense to all.

What I am saying here is that there are a great many factors which can account for the scores obtained and that it is the responsibility of the examiner to be sure the results are an accurate picture of the child's intellectual skills and not an artefact of other influences which mask the true skill levels.

It is only possible to make full sense of test scores if they are stated in full in the assessment written report. It is often the case that the psychologist will only report a range of scores, for example, "Verbal IQ: Average Range", "Performance IQ: Borderline Range" This sort of report writing can raise more questions than answers because sometimes the numbers are at the fringes of a range. For example a score of 90 and a score of 109 are both within the 'Average' range but are both at the extreme range, with one Low Average and one High Average. Without stating the exact numbers, it is impossible to get an accurate picture of the child's level of abilities. I suggest that parents request the complete test data, (the actual numbers themselves) – it will be a useful means to compare results if an assessment is re-administered sometime in the future.


Scale Scores

This brings us to the range of scale scores and what they represent. The Wechsler test is supposed to be a test of intelligence and for these purposes, the three scale scores that are calculated correspond to a range of intelligence 'category', from Gifted to Learning Disabled:


Scale Score IQ            Intelligence Range/Special Ed Categor
130 and above Exceptionally Able/Gifted
90-110      Average (not a special education category)
70-79 Borderline General Learning Disability
50-68   Mild General Learning Disability
35-49   Moderate General Learning Disability
Below 35 Severe/Profound General Learning Disability

You might wonder what happens to those children whose scale scores fall between 80-89. The short answer is that they are not generally eligible for special education services; if their reading of mathematic ability is below the 10th percentile they will be looked after by the learning support teacher. If not, they are deemed to be doing as well as other children and will not receive any specialist assistance.

As has been stated earlier, observations are a helpful source of information but it must be remembered that all observations are subjective (liable to be distorted by hidden bias and differing levels of tolerance for learning differences and differences in behavioural skills). In any assessment the sole reliance upon observation and teacher-made tests is inappropriate. Assessment instruments that generate quantifiabledata are a necessary part of the assessment process.



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