EVALUATING CORROBORATIVE EVIDENCE Douglas Walton Department of Philosophy, University of Winnipeg, Canada Chris Reed School of Computing, University of Dundee, UK In this paper, we study something called corroborative evidence. A typical example would be a case of witness testimony evidence. One witness testifies she saw the accused leaving the crime scene, but her testimony, by itself, may not be all that strong as evidence. However, when a second witness testifies independently, saying that he also saw the accused leaving the crime scene, this new evidence corroborates the testimony of the first witness, making it stronger as evidence. Corroborative evidence does not always have to be testimonial. It can sometimes be circumstantial, as in the case where I check to see whether my watch is keeping the right time by asking a friend what her watch says. I may think my watch is right, but this may be corroborated by my friend saying that her watch has the same time. Of course, both watches may be wrong, making the point that collaborative evidence is fallible. In this paper we use two tools to study how to evaluate this kind of evidence. 1 One is the argument diagramming system Araucaria (Reed and Rowe, 2005) and the other is the argumentation scheme (Reed and Walton, 2005) 2 . Deploying these tools will lead us to two different methods of evaluating corroborative evidence. 1. Defining Corroborative Evidence Corroboration, more broadly, however, might be taken to apply to any case in which one item of evidence boosts up the value of another item of evidence higher than it was before. The following definitions for ‘corroborate’ are given by the Oxford English Dictionary: 5. To strengthen (an opinion, statement, argument, etc.) by concurrent or agreeing statements or evidence; to make more sure or certain; to support, confirm: said a. of a person; b. of the confirming statement. 1706 POPE Let. to Walsh 22 Oct., I am glad to corroborate [these observations] by some great authorities. 1751 JOHNSON Rambler No. 153 {page}2 My narrative has no other tendency than to illustrate and corroborate your own observations. 1791 Gentl. Mag. 32/1 He appears to have taken uncommon pains to corroborate all his assertions by an appeal to original authorities. 1820 W. IRVING Sketch Bk. I. 88 He recollected Rip at once, and corroborated his story in the most satisfactory manner. 1860 TYNDALL Glac. I. vii. 54 This observation corroborates those of Professor Forbes. 1878 HUXLEY Physiogr. 54 The simple explanation..has been corroborated by subsequent investigators. {dag}6. intr. To concur in testimony. Obs. rare. 1776 G. SEMPLE Building in Water 23 These…nearly corroborate with the Soundings at R. 1 Many of the issues discussed relate to questions studied in a recent paper on argument evaluation (Goddu, 2003), but we have not has space to comment, in order to fit the length requirements. 2 Helpful discussions of how these tools are proving to be important both in artificial intelligence and law are provided in (Reed and Norman, 2003).