Bob has been asked to testify as a lay witness at trial.

Lay testimony, also called factual testimony, is based on what the witness personally saw, heard, did, or otherwise experienced. A lay witness helps the fact finder understand facts by providing firsthand observations, not specialized opinions. Expert witnesses, by contrast, may provide opinions based on specialized education, training, skill, or experience. Option A describes expert testimony. Option C is improper because witnesses generally should not express opinions on the defendant’s guilt or innocence. Option D also goes beyond the usual role of a lay witness because determining validity or evidentiary weight is generally the role of the court or fact finder, unless expert testimony is properly admitted. Therefore, Bob’s primary purpose is to provide factual observations.