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PSYC*3250 Ch 5.pdf

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PSYC 3250
Deborah Powell

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Friday, October 5, 2012 Chapter 5: Scales, Transformations, and Norms - raw scores, which represent simple counts of the behaviours sampled by the test or measuring procedure, do not always provide useful information - it is often necessary to re-express, or transform, raw scores into some more informative scale Scale - the set of scores that might be reported for a test - scales are very often transformations of the original raw scores Transformations - the simplest transformation is one that expresses scores in terms of percentage rather than raw units - this simple transformation has 3 characteristics than many of the most useful transformations exhibit: 1. It does not change the personʼs score; it simply expresses that score in different units 2. It takes into account information not contained in the raw score itself-here, the number of items on the test 3. It presents the personʼs score in units that are more information or interpretable than the raw score Linear Transformations - Transformed score = constant + (weight x raw score) - this form of a transformation guarantees a simple, direct relationship between raw scores and transformed scores that can be depicted using a straight line - the z transformation is extremely useful for a number of reasons - first, the z score indicates each personʼs standing as compared with the group mean - when the distribution of raw scores is reasonably normal, z scores can be directly converted to percentiles - z scores are very useful in interpreting most psychological tests and measurements Variations of the Z Transformations - there are 2 features of z scores that are undesirable: half of the z scores in a distribution will be negative and z scores often have fractional values, even when they exceed 11 or 21 - several score transformations can overcome these disadvantages - the T score is a simple transformation of z that takes the following form: - T score - (z score x 10 ) + 50 Area Transformations - uses the normal curve to create scale score values - linear transformations simply change the units of measurement - e.g. changing Fahrenheit to Celsius - rather than simply changing the units of measurement, area transformations change the point of reference - these transformations express a personʼs score in terms of where it falls on a normal curve, rather than simply providing a new unit of measurement Friday, October 5, 2012 - percentile transformation/percentile rank - area transformations do not change the rank order of scores, but they do change the intervals between scores - you can easily convert z scores to percentile ranks - the transformation to percentile ranks is one of the most common and useful transformations of raw scores Stanine - a contraction of the term standard nine and represents one of the simplest transformations available - raw scores are transformed to a single digit between 1 and 9 Equating Scale Scores - in many instances, it is not possible or desirable to use the same test across different groups of examinees or for the same group of examinees at different times - if the same test is used twice, there are a variety of factors that complicate the interpretation of factors (student remembers the questions) - rather than using the same test twice, educators are likely to use alternative forms of the same achievement test - the question is whether the test published can be sure that both forms are equal in terms of their psychometric characteristics (difficulty) - there is growing interest in the problem of equating scores on different versions of the same test that use different testing technologies Equating Alternate Forms Administered to the Same Examinees - the simplest way to equate 2 alternative forms of a test is to distribute both forms of the test randomly to a large representative sample of examinees - random assignment is crucial to accuracy - the next step is to generate the descriptive statistics on both forms of the test - finally, a straightforward linear transformation (z scores) can take place, equating a specific raw score from one test to the scale score that it would equal on the second test Equating Alternative Forms Administered to Different Examinees - we actually have 3 tests: the old test, the new set, plus the common set of items given to both groups of people - the mechanism for equating the new test for to the old test form is through this common collection of item,s referred to as an anchor test - the information obtained from the anchor test provides data that allows one to equate the 2 test forms - if people perform differently on test forms A and B, it might be difficult to determine whether this outcome reflects differences in people taking the test or differences in the tests themselves - the use of a common anchor test, which is taken by both groups of examinees, allows us to separate differences among examinees form differences between the test forms Norms - scores on psychological tests rarely provide absolute, ratio scale measures of psychological attributes Friday, October 5, 2012 - it rarely makes sense to ask how much intelligence, motivation, depth perception a person had - scores on psychological tests do provide useful relative measures - it makes sense to ask if Scott is more intelligent than Peter - one of the most useful ways of describing a personʼs performance on a test is to compare his or her test score with the test scores of some other person or group - a personʼs score is interpreted by comparing that score with the scores of several other people, this comparison is referred to as a norm-based interpretat
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