506. Some electoral or procedural laws (as in Colombia and Mexico) provide a definition of the means of proof so that litigants are aware of the types of evidence that they may offer in support of their factual and legal arguments and claims, and in some cases even of the effect or weight the EDRB should attribute to them. However, most procedural electoral codes and laws do not. Some provide that the codes of civil or administrative procedure may apply unless otherwise stated (for example, those of Bolivia, Guatemala and Paraguay). Many countries require that the relevant documentary evidence be attached to the petition, claim or complainant brief that puts forward the challenge and to the defendant’s or respondent’s plea (as in Chile, Costa Rica, Ecuador, Mexico and Peru). 507. While several systems expressly provide that the parties can offer any means of proof, some establish restrictions, allowing for example some or all of: • electoral documents; • public and private documents, including the official documents related to the action or decision that is being challenged; • admissions, testimony, expert evidence and circumstantial evidence; and • what is called legal and human presumption. All these may be admitted with specified restrictions – for example, that the evidence should be pertinent to the case. 508. In most countries, the EDRB is authorized to demand the introduction of additional evidence in order better to understand the facts at issue, without implying any movement away from the principle that the party making an allegation is obliged to prove it. 509. Because of the short time frames for ruling on electoral challenges, where there are no laws or regulations relating to means of proof, EDRBs have had to develop criteria for admitting suitable or appropriate evidence in support of the parties’ claims within the legal time limits for making a ruling. Witness testimony may for example be restricted, if not excluded, in cases where election results are contested. c) Systems for weighing evidence 510. According to the predominant doctrine of procedural or evidence law, the systems for assessing evidence may be classified into four groups: a. systems of legal or legally weighted evidence, in which the law indicates in advance the effect or weight that the EDRB should attribute to a means of proof; 166