Unless the court orders otherwise, an expert may state an opinion - and give the reasons for it - without first testifying to the underlying facts or data. But the expert may be required to disclose those facts or data on cross-examination.
W.va. R. Evid. 705
COMMENT ON RULE 705
Rule 705 is taken verbatim from tis federal counterpart. The revised rule is substantively the same as the current state rule and the changes are merely stylistic.