                modifications, in determining whether respondent had abused the child.
                Appellant argues that there was substantial evidence to support the
                allegations of respondent's abuse. Appellant also summarily argues that
                the district court expressed bias against her at trial.
                            A court may modify primary physical custody when there has
                been a substantial change in circumstances affecting the child's welfare,
                and the child's best interest is served by the modification.   Ellis v. Carucci,
                123 Nev. 145, 150-51, 161 P.3d 239, 242-43 (2007). Generally, in
                establishing the child's best interest, the court applies a preponderance of
                the evidence standard.     See Mack v. Ashlock,    112 Nev. 1062, 1066, 921
                P.2d 1258, 1261 (1996); but see NRS 125C.230(1) (providing that clear and
                convincing evidence of domestic violence creates a rebuttable presumption
                that custody with the perpetrator is not in the child's best interest). In
                evaluating a custody order, this court must be satisfied that the district
                court's decision was made for appropriate reasons and that the factual
                determinations are supported by substantial evidence.      Rico v. Rodriguez,
                121 Nev. 695, 701, 120 P.3d 812, 816 (2005). Child custody matters rest in
                the district court's sound discretion, and this court will not disturb the
                custody decision absent an abuse of that discretion.      Wallace v. Wallace,
                112 Nev. 1015, 1019, 922 P.2d 541, 543 (1996); Sims v. Sims, 109 Nev.
                1146, 1148, 865 P.2d 328, 330 (1993).
                            Here, the district court determined that the child abuse
                allegations against respondent were unsubstantiated by social services
                and that the district attorney's office declined to bring criminal charges
                against respondent. The court considered conflicting testimony from
                professional therapists and determined that appellant did not show by a
                preponderance of the evidence that respondent had abused the child or


SUPREME COURT
        OF
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that it was in the child's best interest to grant appellant sole legal and
physical custody. It is the duty of the trier of fact, not an appellate court,
to weigh the credibility of witnesses. Castle v. Simmons, 120 Nev. 98, 103,
86 P.3d 1042, 1046 (2004). Having reviewed the record, we conclude that
the district court was fair and unbiased at trial and applied the correct
evidentiary standard. We further conclude that the district court's
findings are supported by substantial evidence and that the district court
did not abuse its discretion in its custody determination.      See Rico, 121
Nev. at 701, 120 P.3d at 816; Wallace, 112 Nev. at 1019, 922 P.2d at 543.
Accordingly, we
            ORDER the judgment of the district court AFFIRMED.




cc: Hon. Chuck Weller, District Judge, Family Court Division
     Sandra A. Unsworth
     Jerod Arreguini
     Washoe District Court Clerk




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