Is a judge's completely arbitrary determination of credibilty subject to appeal?

Question

If a United States Federal judge credits critical testimony without fully explaining him or herself, is it subject to appeal?

For example, suppose a judge awards an amount to the winning party because "I found witness C's testimony on X to be credible", would there be grounds to appeal if there are good arguments to be made why C's testimony about X is not credible (say C contradicted himself in other places, etc)

Answer 1

First: see How is a judge to evaluate a witness's credibility?

Appealability of credibility findings is extremely limited

A fact-finder's determination of credibility is not an appealable issue unless it can be traced to:

• an error of law (e.g. if a judge found that a complainant is less worthy of belief because of his or her sexual history); or
• what is known as a "palpable and overriding error" in Canada or "clear error" in the United States (e.g. the judge based an adverse credibility finding on a piece of evidence that just didn't exist — like the judge writing that the witness said X and Y, but the transcript shows that the witness never said Y).

See footnote 141 of Moffat v Edmonton (City) Police Service, 2021 ABCA 183 for a good summary of the standard in both the US and Canada.

But as long as there is no error of law and no palpable and overriding error (or clear error), credibility findings will not be overturned on appeal. A reviewing court will not simply replace the judge's credibility finding because they would have found differently.

Credibility findings are not all or nothing

Also, credibility findings do not necessitate accepting all or none of a witness's testimony. This is made explicit in typical jury instructions:

you may disregard so much of the testimony as you find was untruthful, and accept so much of it as you find to have been truthful and accurate

Less than full explanation is not the same as completely arbitrary

Last, given your title and introductory paragraph, it seems you may be conflating "completely arbitrary" and not "fully explaining." These are wildly different circumstances. The adequacy of a trial judge's reasons is assessed on a functional basis. The Supreme Court of Canada has said (and this approach is similar elsewhere):

The question is whether the reasons are sufficient to allow for meaningful appellate review and whether the parties’ “functional need to know” why the trial judge’s decision has been made has been met. The test is a functional one.

In determining the adequacy of reasons, the reasons should be considered in the context of the record before the court. Where the record discloses all that is required to be known to permit appellate review, less detailed reasons may be acceptable. This means that less detailed reasons may be required in cases with an extensive evidentiary record, such as the current appeal. On the other hand, reasons are particularly important when “a trial judge is called upon to address troublesome issues of unsettled law, or to resolve confused and contradictory evidence on a key issue”, as was the case in the decision below. In assessing the adequacy of reasons, it must be remembered that “[t]he appellate court is not given the power to intervene simply because it thinks the trial court did a poor job of expressing itself”.

Less than a full explanation does not turn a credibility determination into a completely arbitrary one (e.g. accepting testimony based on a flip of a coin).

Answer 2

Findings of fact are generally not appealable*

^{* - as long as they are not obviously unreasonable.}

A finder of fact - be it judge or jury - is allowed to weigh credibility as they see fit as long as the result is not totally unreasonable and has any foundation in the testimony and evidence. Contradictions on one topic do not necessarily make the witness in total not credible. It's the same standard as in your previous question, and the same standard applies.

For something that could be appealed, the outcome needs to be far out of the believeable and totally unsupported by any evidence. The Court of Appeals has to give deference to any findings of fact in the previous instance that are reasonable and may only discard findings that are unreasonable and unsupported by admissible evidence.

• Assume Alice is charged with wearing blue pants. All witnesses (including Alice) state that Alice wore red pants. The Pants in evidence are red. Yet the finder of fact finds Alice wore blue pants based on no evidence. That can be appealed and overturned for a retrial.
• If in the same case there is even a single piece of evidence or witness that says Alice wore blue pants, and the judge/jury believe that one witness over all others, that is nigh impossible to appeal
  • as long as that testimony is not clearly wrong
  • or inadmissable at that trial. Inadmissibility of evidence (among others: fruit of the poison tree doctrine) can open a question of law review, which in turn can put the finding of fact on shaky grounds later.
• Still the same charges and same testimony about red or blue pants, but the finder of fact decides that Alice is a dog. That is clearly unreasonable and not even charged, and that verdict should automatically bring a mistrial.

The Standard here is, as Jen notes, "palpable and overriding error", which is comprised of two prongs:

• palpable - The error or logic followed need to be palpable, which means obvious.
  • In the first example, it's obvious the pants were not blue, so the error is obvious. In the variation, it's unclear which of the witnesses are right, so it would be not obvious.
• overriding - The error needs to impact the result of the case.
  • In the examples this was ensured by the setup.