What is the test for a modification of a child support order for abuse of discretion?

We review orders granting or denying a request for modification of a child support order for abuse of discretion. (Brothers v. Kern (2007) 154 Cal.App.4th 126, 133, 64 Cal.Rptr.3d 239; In re Marriage of Pedrlstein (2006) 137 Cal.App.4th 1361, 1371, 40 Cal.Rptr.3d 910.) The trial court's exercise of its discretion must be "informed and considered" (In re Marriage of Muldrow (1976) 61 Cal.App.3d 327, 332, 132 Cal.Rptr. 48; see In re Marriage of Pearlstein, at p. 1371, 40 Cal. Rptr.3d 910), and the trial court may not "ignore or contravene the purposes of the law" (County of Stanislaus v. Gibbs (1997) 59 Cal.App.4th 1417, 1425, 69 Cal.Rptr .2d 819).