NiallNai

Generally, it is fairly true but there is room and causes for variance. Depending on the standards that are used during the scoring tests, there is a range in which the person's IQ is expected to fluctuate if they were give a test again. These tests are better seen as a snapshot of performance on a particular day and are sensitive to a number of things.

People with depression often score lower on tasks that involve reaction time and memory. So you might see a drop in IQ due to that.

Also, as the norms for tests age the average scores of the test takers increase due to the information on the test coming into greater use in the general population.

I've also seen IQs drop drastically when a new edition of a test comes out. When the WISC-IV (the IQ test used on the majority of school age children, at least in New Jersey) replaced the WISC-III, I saw a number of students scores drop by 5 to 10 points between the two tests. This is both because of the new norms and something else that I'm not sure about.

Also, as people age, different tests may be used. The WPPSI-III is for pre-schoolers and goes up to age 6 (If memory serves; I work largely with high school students); the WISC-IV is for about 6 to 17; the WAIS-III (soon to be IV) is for 17 to about 89+. So while the tests have been developed with an eye toward making them consistent since they are all created by the same company, there are some tasks on the WPPSI that aren't on the WAIS and vice versa, so you can't necessarily say with 100% accuracy all three tests are measuring the exact same things.

Now if you use the Stanford-Binet-V or the Woodcock-Johnson-III (I'm not making up the name here, let your dirty side run wild with this name, I know I have) Cognitive section, those tests are used from age 2 to 85+, so you might have a greater consistency in scores because the same test is being used throughout the life of the child.

Also, you need to take into account that these tests measure particular abilities that are theorized to be part of the what makes up intelligence. I like to say that if aliens landed on our planet tomorrow and said that you can measure someones IQ with this basic bloodtest, it might make our current IQ tests obsolete.

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