What is the burden of proof in a custody case?

are not persuaded that either party had the burden of proof. Rather, "[t]he trial court is always bound to make a custody decision based upon the child's best interest. But depending upon the posture of the case, the trial court will use either the 'best interest' analysis or the 'changed circumstances' analysis. The best interest analysis is used when making a permanent custody determination initially. 'In an initial custody determination, the trial court has "the widest discretion to choose a parenting plan that is in the best interest of the child." (Fam. Code, 3040, subd. (b).) It must look to all the circumstances bearing on the best interest of the minor child.' [Citation.] [] The changed circumstances test requires a threshold showing of detriment before a court may modify an existing final custody order that was previously based upon the child's best interest." (Ragghanti v. Reyes (2004) 123 Cal.App.4th 989, 996.)