Under RSA 542:8, arbitration awards can be corrected or modified for "plain mistake" either of fact or of law. Correction or modification for misapplying the law is available if the mistake was one which was "apparent on the face if the records and which would have been corrected had it been called to the arbitrator's attention." Merrill Lynch Futures, Inc. v. Sands, 134 N.H. 507, 509 (1999). That's a bit more deferential than review of a trial court's errors of law.
Appellate review of an arbitrator’s mistakes of fact is somewhat more limited as well. When a trial court or jury finds facts based on the evidence presented, the Supreme Court defers to those findings and will not “retry” the case as long as there is some evidence apparent on the record to support the trial judge’s findings. Review of an arbitrator’s findings of fact is similarly limited; the courts will “defer to the arbitrators' decision if the record reveals evidence supporting it,” Merrill Lynch Futures, Inc. v. Sands, 143 N.H. 507, 509 (1999). But there is an extra layer of deference to arbitrators’ findings not accorded to a judge or jury. The Supreme Court “will set aside a jury verdict if it is conclusively against the weight of the evidence . . . Conclusively against the weight of the evidence should be interpreted to mean that the verdict was one no reasonable jury could return.” Quinn Bros. v. Whitehouse, 144 N.H. 186, 190 (1999). Not so an arbitration award; a court will not “set aside the decision merely because it believes the arbitrator’s award is against the weight of the evidence,” Masse v. Commercial Union Ins. Co., 136 N.H. 628, 632 (1993).
Because "arbitration is a matter of contract and a party cannot be required to submit to arbitration any dispute which he has not agreed so to submit," Appeal of Merrimack County Bd. of Commissioners, 142 N.H. 768, 771 (1998), reversal on appeal is also possible if the arbitrator exceeded his or her authority by deciding a matter not within the scope of the arbitration agreement. “A judicial challenge to arbitral authority requires the reviewing court to consider both the contract and the arbitral submission,” Lebanon Hangar Associates, Ltd. v. City of Lebanon, 163 N.H. 670, 673 (2012). “Moreover, an arbitrator's view of the scope of the issue is entitled to the same deference normally accorded to the arbitrator's interpretation of the contract.” Id. While some arbitration clauses are broader than others, “In the absence of clearly restrictive language, great latitude must be allowed in the framing of an award and fashioning of an appropriate remedy,” John A. Cookson Co. v. New Hampshire Ball Bearings, Inc., 147 N.H. 352, 361 (2001). Courts have taken this to heart. The boilerplate arbitration provision in the AIA General Conditions form A201 (1997 version), referencing arbitration of “[a]ny claim arising out of or related to the Contract,” has been almost universally held to require the contracting parties to arbitrate everything under the sun – including the question of whether a demand for arbitration was timely filed (the most common area of attack on an award for allegedly exceeding the arbitrator’s powers).
The final basis for attacking an arbitration award is for fraud, corruption, or misconduct of the arbitrator. Since arbitrators are an honest lot, and are typically picked by the parties after they disclose any interest or familiarity they may have with the case or the parties, it will be a rare day when this comes into play. Suffice it to say that the New Hampshire Supreme Court has never once overturned or affirmed the overturning of an arbitration award on this basis.
If you’re getting the idea that an arbitration award is close to appeal-proof and overwhelmingly likely to be affirmed, you’re getting the right idea!