In additional to proving there were negligent mistakes, a plaintiff in a legal malpractice case must prove that the attorney mistakes caused damage. In a case in which the malpractice involved the mishandling of an underlying claim or lawsuit, this means that generally the plaintiff must prove, more likely than not, what he or she would have recovered if the claim or lawsuit had been handled properly.
Such questions of causation and damages are usually the exclusive province of the fact finder (jury) to determine, not the judge presiding over a jury trial by ruling on a motion for instance, unless jury has been waived. The malpractice jury is supposed to determine what a reasonable jury in the underlying case should have done if the case had gone to trial and had been properly presented. Griswold v Kilpatrick, 107 Wn. App. 757, 27 P.3d 246 (2001); Daugert v Pappas, 104 Wn.2d 254, 258, 704 P.2d 600 (1985); Brust v. Newton, 70 Wash.App. 286, 852, P.2d 1092 (1993).
Except in unusual circumstances, for causation and damages, the malpractice jury considers the same kind of evidence the jury in the underlying case would have considered. The case-within-a-case should be presented as it would have been or should have been in the first instance. The malpractice jury simply acts as if it were the jury in the underlying case, either hearing for the first time a case that never made it to trial before, or hearing a previously-tried case with evidence and arguments that were negligently omitted the first time around, coupled with a transcript of the earlier trial.
The malpractice jury, in deciding how the case-within-a-case would have fared without negligence, in a case that was already tried to a jury for instance, is not supposed to guess what the earlier jury actually would have done with the missing evidence. Chocktook v Smith, 280 Or 567, 571 P.2d 1255 (1977). Thus, malpractice counsel cannot depose or subpoena for trial the jurors from the underlying case, present them with the missing evidence, and ask it if would have changed their minds. In this sense, it is sometimes said that the malpractice jury is to decide, not what the earlier jury would have done, but what it should have done. (Watson v Meltzer, 270 P.3d 289 (2011) seems to misinterpret Chocktook in regards to this distinction, but in this context the question is semantic, since Watson does not alter the point here that one does not examine jurors.)
Under a traditional causation analysis (ie, what the jury would actually have done with the missing evidence), the door would be opened to a potential procedural train wreck, in which former jurors are subpoenaed to account for earlier decisions, and asked to opine what they would have done perhaps years earlier in a case as to which they probably have scant memory of the details. Essentially for policy reasons, the courts do not entertain such discovery or testimony.
The proper role of experts in this area is a subject of some confusion and/or opportunism by the defense in a legal malpractice case. See article below.