In writing your report you need to keep in mind the likely reader or readers. Your attorney must be able to understand what you've done, and a Judge must understand it as well. In all likelihood the report will be offered as an exhibit at trial so you need to consider that an average juror will also be reviewing it. Therefore, you do not want to be excessively technical. If technical explanations are required, you need to provide interpretations of the technical matters in lay terms that all of the people reading your report can understand. Define technical terms in the body of the report or with footnotes. Use analogies to explain methodologies and technical aspects of your analysis. In short, avoid writing geek to geek.
Finally, include enough information so your report is intelligible and you have offered a thorough statement of each opinion you intend to offer at trial. Do not, however, include more than is necessary to meet your obligations under the rules. Avoid volunteering information that is not specifically relevant. You are writing a report, not a thesis. If you volunteer too much, all you are doing is providing the opposing attorney with ammunition to use in cross examination. Be cognizant of the fact that whatever you state can and will be used against you if the opportunity arises. Therefore, be as precise and as succinct as you can be.
From: Preparing an Expert Report by Bruce A. Olson