© D.L. Crumbley
An expert witness can state an opinion or conclusion if four conditions are satisfied:
The validity of the opinion or conclusion depends on special knowledge, experience, skill, or training not ordinarily found in lay jurors;
The witness must be qualified as an expert in the pertinent field;
She must possess a reasonable degree of certainty (probability) about her opinion or conclusion; and
Generally, in common law jurisdictions an expert must first describe the data on which her conclusion is based, or she must testify in response to a hypothetical question that sets forth such data.
Source: J.R. Waltz, Evidence, New York: Foundation Press, 1999, p. 15.