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The reason for the choice of the number 21.06 is to bring about the following result: If the scores are normally distributed (i.e. they follow the "bell-shaped curve") then the normal equivalent score is 99 if the percentile rank of the raw score is 99; the normal equivalent score is 50 if the percentile rank of the raw score is 50;
Percentile ranks are not on an equal-interval scale; that is, the difference between any two scores is not the same as between any other two scores whose difference in percentile ranks is the same. For example, 50 − 25 = 25 is not the same distance as 60 − 35 = 25 because of the bell-curve shape of the distribution. Some percentile ranks ...
Like stanines, individual sten scores are demarcated by half standard deviations. Thus, a sten score of 5 includes all standard scores from -.5 to zero and is centered at -0.25 and a sten score of 4 includes all standard scores from -1.0 to -0.5 and is centered at -0.75. A sten score of 1 includes all standard scores below -2.0.
In statistics, a k-th percentile, also known as percentile score or centile, is a score below which a given percentage k of scores in its frequency distribution falls ("exclusive" definition) or a score at or below which a given percentage falls ("inclusive" definition); i.e. a datum in the k-th percentile would be above approximately k% of all data in its set.
Norms provided for the 1978 edition include standard scores with a mean of 100 and a standard deviation of 15, percentile scores, and grade levels. The standard scores are scaled based on the norm group; the grade levels are arbitrarily assigned and can be interpreted only as rough references to achievement level.
A raw score is a score without any sort of adjustment or transformation, such as the simple number of questions answered correctly. A scaled score is the result of some transformation(s) applied to the raw score, such as in relative grading. The purpose of scaled scores is to report scores for all examinees on a consistent scale.
The total score can be converted to a percentile rank, mental age, or a standard deviation IQ score. Although desirable, no special training is required to properly administer and score the PPVT-IV. The test publisher recommends that anyone interpreting or explaining the test scores should have knowledge in psychological testing and statistics.
The University of Alberta in Edmonton, Alberta, Canada used the stanine system until 2003, when it switched to a 4-point scale. [3] In the United States, the Educational Records Bureau (they administer the "ERBs") reports test scores as stanines and percentiles. The New Zealand Council for Educational Research uses stanines. [4]