One of the most important skills a computer forensic investigator must develop is effective testimony as an expert witness.One of a computer forensic expert's most important functions is to serve as a testifying expert in court. For many people, without prior experience as a testifying witness or some other background in the legal system that provides insight into the role of an expert witness at trial, the idea of being a testifying witness can be intimidating or even frightening. Doing a good job in terms of the forensics goes without saying. Writing a thorough and enlightening report is also expected. Ultimately, however, the expert must subject his work and opinions to hostile examination, first in a deposition and later in cross-examination in court.

Direct testimony in which the lawyer who has hired you helps you explain what you've done and how you've derived your expert opinions in the case is relatively easy, though even direct testimony can be a challenge in terms of being able to effectively communicate technical terms and issues in an understandable way to a lay jury. The difficult part is dealing with a hostile cross-examination in which the opposing attorney's job is to make you look inarticulate, incompetent, and ultimately not worthy of belief. There are few circumstances in life where one is subjected to the kind of sustained personal attack that cross-examination can be in the hands of a skillful trial attorney.

The role of the computer forensic expert as a testifying witness in court will be addressed in detail in this series of articles authored by Bruce A. Olson, an experienced board certified civil trial attorney and a CCE who has testified at both depositions and in court as a computer forensic expert. It is hoped that with the information offered in these articles you will be better prepared to meet the challenge of being a testifying expert when the opportunity presents itself.

The articles will focus on the role of an expert in federal court, with reference to the Federal Rules of Civil Procedure and the Federal Rules of Evidence, though in general the same discussion will apply for those testifying in state court. The main difference is that different states have somewhat different evidentiary rules that apply and attempting to identify the rules that apply in each state would be beyond the scope of these articles. In general, however, if you can handle testimony in federal court you should be fine in state court.

The Role of an Expert Witness in Litigation
In order to perform your job well, it is important to understand what the role of an expert witness is in the first place. Basically there are two types of witnesses at trial—lay and expert. Different rules apply to each, and the nature and scope of their testimony is affected by their status. In a nutshell, a lay witness can only testify to facts obtained through personal knowledge or observation. An expert is allowed to go beyond personal knowledge and observation to offer opinions on ultimate issues in the case, provided that certain threshold qualifications are met establishing the witness' specialized expertise. To serve as an expert witness you must first be qualified as such. This process will be discussed in detail in the section on dealing with the Daubert challenge. If you cannot meet the challenge you will not be allowed to testify. Once qualified, however, the scope of you testimony can be much broader and potentially more significant than that offered by a typical lay witness.

Purpose Of Expert Witness
The purpose of an expert witness is really fairly straightforward. The applicable rules are set forth in Rules 701 through 706, Federal Rules of Evidence. As noted in Rule 702, F. R. E.: "If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education may testify thereto in the form of an opinion or otherwise...." Thus, the role of the expert is to assist the jury in understanding the evidence, helping it to make factual decisions on the issues that are involved in a particular case. The distinction between an expert witness and a lay witness is that lay witnesses are limited to offering testimony concerning facts of which they have personal knowledge, and they are not permitted to offer opinion testimony. In contrast, the expert witness can testify regarding the facts relied on, including facts that might otherwise not be admissible in evidence, and then offer opinions on the ultimate issues in the case.

To be clear, just because one holds oneself out as an expert does not mean one can offer unsubstantiated opinion testimony. As Rule 702 F. R. E., goes on to say, the expert may testify only "if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case." How these factors are interpreted and how the expert witness must address each of these categories before offering opinion testimony will be dealt with in detail below.

The expert may base his opinion testimony on facts that are made known to him from a variety of sources. Typically this could include the examination of the electronic evidence using standard forensic techniques, a review of pleadings and depositions in a case, a review of pertinent literature, interviews with other witnesses, and anything else that can shed light on the issues the expert is asked to address.

Furthermore, an expert is allowed to rely on facts that might otherwise not be admissible in evidence. As Rule 703 F. R. E., states:
The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence in order for the opinion or inference to be admitted. Facts or data that are otherwise inadmissible shall not be disclosed to the jury by the proponent of the opinion or inference unless the court determines that their probative value in assisting the jury to evaluate the expert's opinion substantially outweighs their prejudicial effect.

When in court, the expert need not testify to the underlying facts before offering testimony regarding any expert opinions or inferences unless the court requires it. However, the expert may be required to disclose the underlying facts or data relied on in cross examination. See Rule 705 F. R. E.

With this general understanding in mind, the next article will focus on what you need to do to qualify as an expert witness.

Bruce A. Olson is an experienced board certified civil trial attorney and a CCE who has testified at both depositions and in court as a computer forensic expert.