II. Types of witnesses: general witnesses versus experts witness
A. Witnesses of facts
Witnesses of facts, as the name implies, usually provide evidence regarding objective facts that are relevant to the dispute. Witnesses of facts can thus testify, for example, on facts which cannot otherwise be proven by documents or provide further context which documents alone do not provide (see also Fact Witness).4 Usually, witnesses of facts are not permitted to include mere subjective opinion in their statements.5
B. Expert witnesses
Expert Witnesses will usually testify in regard to their own area of expertise, which might be beyond the expertise of the tribunal.6 It is very common to use experts for the quantification of damages, but experts can testify in regard to any field that requires special professional or technical expertise and knowledge that the tribunal does not possess (such as interpretation of national law, issues related to engineering or other scientific issues, valuations of assets, etc.). Contrary to witnesses of fact, it is usually perfectly acceptable for an expert to include her or his relevant professional opinion in the expert report (see also Expert Witness).7
III. Qualification to testify
The IBA Rules on the Taking of Evidence (the "IBA Rules") provide that any person can present evidence as a witness of facts. Contrary to the rules applicable to the use of witness evidence in some civil law jurisdictions, the IBA Rules include the parties themselves, their officers, employees or representatives.8
As to expert witnesses, there is no minimum requirement of qualifications for an expert to be permitted to testify as such. Article 5 of the IBA Rules provides that experts appointed by the parties should provide a description of his or her background, qualifications, training and experience.9 Usually the arbitral tribunal will assess whether the expert indeed possesses the required qualification, knowledge and expertise relevant to the issues on which he or she is to opine based on the information provided by the expert and information that emerges during his or her cross-examination.10
IV. Witness statements and experts' reports
Witness statements will usually include the name and address of the witness, her or his relationship with the party on behalf of whom the statement was submitted, a full description of the facts, sources and documents on which the witness relied in their testimony and a signed confirmation of truth.11
Experts report will usually include a description of the expert's relationship with the party, professional background and a statement of independence followed by the expert's opinion, conclusions and the sources and methodology used to reach them.12
V. Who can call a witness to testify?
Each party can decide who to call as a witness. In practice, the tribunal will usually set a timeframe for the parties to identify their witnesses (both witnesses of facts and experts) and to submit witness statements and expert reports. A party wishing to call a witness who will not appear for testimony voluntarily can do so by requesting that the tribunal take the legally available steps to obtain the testimony.13
Khodykin, R., Mulcahy, C. and Fletcher. N., Commentary on the IBA Rules on Evidence, Article 4 [Witnesses of Fact], in A Guide to the IBA Rules on the Taking of Evidence in International Arbitration, 2019, pp. 214-278.
Khodykin, R., Mulcahy, C. and Fletcher. N., Commentary on the IBA Rules on Evidence, Article 5 [Party Appointed Expert], in A Guide to the IBA Rules on the Taking of Evidence in International Arbitration, 2019, pp. 279-314.
Sourgens, F. O., Duggal, K. and Laird, I., Witnesses and Experts, in Evidence in International Investment Arbitration, 2019, pp. 196-234.
Waincymer, J., General Witness and Expert Evidence, in Procedure and Evidence in International Arbitration, 2012, pp. 885-976.
Niemoj, A., The Limitations on Article 43 ICSID Convention: An (Un)limited Instrument of the Tribunal?, ICSID Review - Foreign Investment Law Journal, pp. 697–722.
Harbst, R., Witness Statements, in A Counsel's Guide to Examining and Preparing Witnesses in International Arbitration, 2015, pp. 97-96.
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