Rule 26(a)(2)(B) of the Federal Rules of Civil Procedure provides that a party must disclose to other parties the identity and opinions of each testifying expert.  The expert must provide a written report to the opposing party. The Federal Rule states that

" the report shall contain:

1. a complete statement of all opinions to be expressed and the basis and reasons for the opinion;
2. the data or other information considered by the witness in forming the opinions;
3. any exhibits to be used as a summary of or support for the opinions,
4. the qualifications of the witness, including a list of all publications authored by the witness within the preceding ten years;
5. the compensation to be paid for the study and testimony;
6. a listing of any other cases in which the witness has testified as an expert at trial or by deposition within the preceding four years." 

   Rule 26(a)(2)(B) of the Federal Rules of Civil Procedure [Emphasis supplied].

Since Rule 26(a)(2)(B) requires that the expert witness's report include a "complete statement" of "all opinions to be expressed," plus a "complete statement" of the "basis and reasons" for each opinion.  The expert's report must be reasonably comprehensive.  The reach of this Federal Rule  language is uncertain; it is left to the court to decide how much detail is required to be a "complete statement".

 See material on Federal Rule 702 of the Evidence Rules.