The Wechsler-like IQ subtest scaled score metric: The potential for misuse, misinterpretation and impact on critical life decisions---draft report in search of feedback

I've recently been skimming James Flynn's new book (What is Intelligence: Beyond the Flynn Effect) to better understand the methodology and interpretation of the Flynn effect. Of particular interest to me (as an applied measurement person) is his analysis of the individual subtest scores from the various Wechsler scales across time. As most psychologists know, Wechsler subtest scaled scores (ss) are on a scale with a mean (M) = 10 and a standard deviation (SD) = 3. The subtest ss range from 1 to 19. In Appendix 1 of his book, Flynn states "it is customary to score subtests on a scale in which the SD is 3, as opposed to IQ scores which are scaled with SD set at 15. To convert to IQ, just multiply subtest gains by five, as was done to get the IQ gains in the last column." At first glance, this statement makes it sound as if the transformation of subtest ss to IQ SS is an easy just and mathematically acceptable procedure without problems. However, on close inspection this transformation has the potential to introduce unknown sources of error into the precision of the transformed SS scores. It is the goal of this brief technical post to explain the issues involved when making this ss-toIQ SS conversion.