Testing is good for two main things: unlocking access to services, and finding out more about your child to better raise him. If your son is highly gifted, raising him will be a significantly different experience from raising a normal or moderately gifted child. Full testing, if it goes well, will probably give you some useful information, may highlight areas of difficulty, etc. And if you go to a tester that specializes in gifted children, they will probably give you a lot of insight on how to approach raising your son, if he does turn out to be gifted.
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Now in your case, the testing might be extra useful because of the ADHD diagnosis. Gifted children may suffer from ADHD, but also be misdiagnosed with it too (for example, out of boredom with things in which they're not interested, which might be the case with your son even if he's not openly showing it in ways you understand). The testing might help explain some of his cognitive differences in a way that sheds more light.Â
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Testing can be done in a variety of ways. You would have to pick a tester, and there is a full gamut from gifted testing experts to freebie interns at the local college. Here, you sometimes get what you pay for, and you should make sure that your tester, whoever it is, has tested gifted children before and knows their quirks.Â
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You also have to pick one or more tests. If you're going to engage in testing, it is probably worthwhile to do both a cognitive/IQ type of test and a more achievement-oriented test. The most common, and commonly accepted, IQ tests for children are the Stanford-Binet 5 (SB5) and the Wechsler Intelligence Scale for Children (WISC-IV), followed by the WPPSI, which is used generally for very young children. For children under 5, if you want to go with the biggies, you're pretty much stuck with either the SB5 or WPPSI. Once a child hits 6 and the WISC-IV is available, the WPPSI is not considered to be the best option; pick from the SB5 or WISC.
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There are different achievement tests too, and the most common seem to be the Woodcock-Johnson III (WJ-III) and WIAT. Your tester will educate you on the differences between all of these tests, and may steer you toward particular ones based on having more familiarity with, liking for, or access to particular tests.
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On the services end, having documentation can help with placement in gifted and talented services. If your son turns out to be exceptionally or profoundly gifted, some programs like the Davidson Young Scholars program can be beneficial in giving access to further services, free advocacy, etc. Good luck.