The extended scores don't seem to be needed, but fwiw, we haven't ever been provided with extended scores for our ds and I always wished I would have asked for them in the report just to have them. Even if no one is ever going to need to see them or use them, I think it's a good idea to have them documented if you can simply because some day in the future you might want them, and it might be too late then to call up the original testing psych to ask for them.
If I remember the coding subtest correctly, it is a series of incomplete symbols that you have to mark based on a key to make the symbols match the key. It's a simple dash-type mark, and it's timed. Each of the WISC subtests seems to be a relatively short test, so from your dd's description of it, I think it's most likely she was simply impacted by a combination of not being a person who races by nature and worrying over the marks she felt she hadn't made correctly. Unless at some time in the future you feel that her rate of performance on timed tasks or with handwritten tasks etc is negatively impacting her ability to complete her school work or to show her knowledge, I wouldn't ever think twice about that score ever again
I *would* ask the tester if they could put a note about what your dd said (worrying about the incorrect answers) in the report so that in the future if someone looking at her scores wonders about the difference, there will be a likely explanation documented.
"I am also wondering what exactly the coding subtest is measuring. If two persons have the exactly same scores in the rest subtests except a big difference in PSI, more specifically coding subtest, what would these two persons be different in their learning or academic life?"
The difference in their learning/academic life is really going to depend on what impacted the difference in score. If vision impacted it, they are going to have impacts in tasks that require vision skills; if it's fine motor related, things like handwriting speed etc will be impacted. If the lower score is due to a person who simply moves a bit slower with their handwriting, perhaps they approach their school work a tiny bit slower than a person with a rapid-fire coding speed. You noted that your ds with the higher PS score works faster than your dd - that seems to fit your description of how your dd approached the coding subtest. JMO, but I don't think that's necessarily anything to worry about - there is something to be said for taking a careful, well thought-out approach to your work
One of my dds has an extremely high PS as well as a high WM score, and she seems to be working at light-speed compared to my other kids, but she also misses details thanks to her need for speed
Best wishes,
polarbear
