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IQ classification is the practice by IQ test publishers of labeling IQ score ranges with category names such as “superior” or “average”. IQ scores have been derived by two different methods since the advent of cognitive ability tests. The current scoring method for all IQ tests is the “deviation IQ”. In this method, an IQ score of 100 means that the test-taker’s performance on the test is at the median level of performance in the sample of test-takers of about the same age used to norm the test. An IQ score of 115 means performance one standard deviation above the median, a score of 85 performance one standard deviation below the median, and so on. Even before IQ tests were invented, there were attempts to classify people into intelligence categories by observing their behavior in daily life. All IQ tests show variation in scores even when the same person takes the same test over and over again.
IQ scores also differ for a test-taker taking tests from more than one publisher at the same age. IQ scores can differ to some degree for the same person on different IQ tests, so a person does not always belong to the same IQ score range each time the person is tested. IQ tests generally are reliable enough that most people ages ten and older have similar IQ scores throughout life. Still, some individuals score very differently when taking the same test at different times or when taking more than one kind of IQ test at the same age. Because all IQ tests have error of measurement in the test-taker’s IQ score, a test-giver should always inform the test-taker of the confidence interval around the score obtained on a given occasion of taking each test. IQ scores are ordinal scores and are not expressed in an interval measurement unit. IQ classifications for individuals also vary because category labels for IQ score ranges are specific to each brand of test.
The test publishers do not have a uniform practice of labeling IQ score ranges, nor do they have a consistent practice of dividing up IQ score ranges into categories of the same size or with the same boundary scores. IQ classifications from IQ testing are not the last word on how a test-taker will do in life, nor are they the only information to be considered for placement in school or job-training programs. There is still a dearth of information about how behavior differs between persons with differing IQ scores. The lesson here is that classification systems are necessarily arbitrary and change at the whim of test authors, government bodies, or professional organizations.