The short answer
Lay witnesses testify about what they saw or heard. Expert witnesses, qualified by knowledge or experience, may give opinions on technical issues such as causation or damages.
Where it fits in your case
Experts are disclosed during discovery and testify at trial. Disputes over their opinions are often decided before trial.
What to expect
Disclosure. Illinois Rule 213(f) requires disclosure of each expert’s opinions and bases, and testimony is generally limited to what was disclosed.
Admissibility. Illinois applies the Frye general-acceptance test to new scientific methods; federal courts apply Daubert.
Changed opinions must be disclosed promptly or may be barred.
Causation. An expert who cannot pinpoint the exact cause may still testify if the opinion rests on a reliable basis.
What we need from you
Technical documents, access to the people who understand them, and early decisions about expert budgets.
Common misconceptions
“Any knowledgeable employee can be our expert.” Retained and employee experts face different rules.
“Experts decide the case.” Jurors can reject expert opinions.