More than one audience. You're likely to find that your report is for more than one audience. Wide variability in an audience. You may realize that, although you have an audience that fits into only one category, there is a wide variability in its background. But if you don't write to that lowest level, you lose that segment of your readers.
Most writers go for the majority of readers and sacrifice that minority that needs more help.
Others put the supplemental information in appendixes or insert cross-references to beginners' books. The most difficult situation to face as a writer is the unknown audience. The obvious advice is to spend whatever time or resources you need in order to find out all you can about your audience's background. This should be part of your background research as you prepare any report. However, despite your best efforts, there still may be times when you find that you must write to an unknown audience.