The subject matter must be within the field of expertise of the expert witness and the fact finder must lack the special knowledge to be informed. The modern trend allows testimony to the ultimate issues in the case as well as to the subject matter. In order to allow a witness to testify as an expert, the court must be convinced that the witness will rely only upon evidence a reasonable expert in the field would rely. United States v. Lundy, 809 F.2d 392, 395 (7th Cir. 1987).