We review for an abuse of discretion. Pratt v. Piper, 500 N.W.2d 716, 716 (Iowa App. 1993). In order to show an abuse of discretion, one generally must show that the court exercised its discretion on grounds or for reasons clearly untenable or to an extent clearly unreasonable. Id. In ruling upon motions for new trial the trial court has a broad but not unlimited discretion in determining whether the verdict effectuates substantial justice between the parties. Iowa R. App. P. 14(f)(3). the court is slower to interfere with the grant of a new trial than with its denial. Iowa R. App. P. 14(f)(4).