Half a standard deviation means going from the 50th percentile of IQ to the 69th. I'm assuming you didn't mean statistical significance since size of movement h

Half a standard deviation means going from the 50th percentile of IQ to the 69th. I'm assuming you didn't mean statistical significance since size of movement has nothing to do with statistical significance, so you must have meant significant in the sense of important. (Just saw your rebuttal about "significance" below. Statistical significance is not how big a movement is, it is the chance that such a movement could be due to random chance. A very small movement could be statistically significant if your sample is big enough to show it's probably (e.g. 95% probably) not due to chance.)