Strictly in my opinion, you should not expect any practice exam to perform like an actual CISSP exam. Questions on the exam are composed to ensure your knowledge of the material, and that you read each question thoroughly.
It's been five years for me, but Mike Chapple's practice tests were the most comparable. Still, they were not the same. More to the point, I found no vendor that came closer.
Before doing any practice tests, study. Then study more. Then study even more. Take a practice test, and if you're not anywhere near 85%, make flash cards and study even more, and wait thirty days before taking another practice test.
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A claim is as good as its veracity.
