When experts testify at trial in federal court, their opinions must have a sufficient basis. But sufficient does not necessarily mean admissible. Engineers and architects do not bring the Federal Rules of Evidence with them to the job site. Doctors rely on hearsay from other doctors, patients, and family members. Mathematicians postulate and theorize in forming their judgments, relying in some cases on ancient work passed down from teacher to student for millennia. In scientific and technical fields, reliance on information that would not be properly admitted in a court of law is often necessary and is frequently appropriate.
This begs the question: Can an otherwise qualified expert take the stand and render an otherwise qualified opinion at trial when that opinion is based entirely on inadmissible evidence? Under the Federal Rules of Evidence, the answer is yes. This follows from the text and interplay of Rules 703 and 702.