[EMAIL PROTECTED] (Robert J. MacG. Dawson) wrote in news:[EMAIL PROTECTED]:
>>let's say we have SAT scores where the mean is about 500 and the SD >>about 100 ... here ... the COV is 100/500 = 20% > > IF this were true, I can conclude: the vast majority of scores will > be above 300, with the result that a large proportion of the students' > test-writing time is spent on questions that pretty nearly everybody > gets and which thus have little predictive power. This might justify > a major revision of the testing protocol (or at least the admission > that the goal of making sure that nobody feels squashed is more > important!) It is a That's an urban legend. The reason SAT scores bottom out at 200 is the same reason they top out at 800, namely that there aren't enough test items to meaningfully distinguish levels of aptitude/achievement/whatever way out in the tails. There aren't enough items to distinguish 600 levels of performance, let alone 1000. SAT scores aren't linear functions of the number of correct test items: how many points each item is "worth" varies with the total score; the marginal gain from getting a single test item right increases with distance from the mean (I vaguely recall that the difference between a 750 and an 800 is one test item). SAT scores are just rescaled Z scores (SAT=100*Z+500), with the complication that the mean subtracted and the standard deviation divided by are those of a reference group rather than one's fellow test-takers (IOW, it's normed only once every 50 years or so, not with each administration). So a score of 300, which means nothing more than "the test-taker's raw score was 2 standard deviations below the mean," actually works out to getting very few items right. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
