The verdict may be vacated and any other decision may be modified or vacated, in whole or in part, and a new or further trial granted on all or part of the issues, on the application of the party aggrieved, for any of the following causes, materially affecting the substantial rights of such party:
When a new trial is granted, on all or part of the issues, the court shall specify the ground or grounds upon which it is granted and the court's reason or reasons for granting the new trial upon each ground stated.
A new trial shall not be granted upon the ground of insufficiency of the evidence to justify the verdict or other decision, nor upon the ground of excessive or inadequate damages, unless after weighing the evidence the court is convinced from the entire record, including reasonable inferences therefrom, that the court or jury clearly should have reached a different verdict or decision.
The order passing upon and determining the motion must be made and entered as provided in Section 660 and if the motion is granted must state the ground or grounds relied upon by the court, and may contain the specification of reasons. If an order granting such motion does not contain such specification of reasons, the court must, within 10 days after filing such order, prepare, sign and file such specification of reasons in writing with the clerk. The court shall not direct the attorney for a party to prepare either or both said order and said specification of reasons.
On appeal from an order granting a new trial the order shall be affirmed if it should have been granted upon any ground stated in the motion, whether or not specified in the order or specification of reasons, except that (a) the order shall not be affirmed upon the ground of the insufficiency of the evidence to justify the verdict or other decision, or upon the ground of excessive or inadequate damages, unless such ground is stated in the order granting the motion and (b) on appeal from an order granting a new trial upon the ground of the insufficiency of the evidence to justify the verdict or other decision, or upon the ground of excessive or inadequate damages, it shall be conclusively presumed that said order as to such ground was made only for the reasons specified in said order or said specification of reasons, and such order shall be reversed as to such ground only if there is no substantial basis in the record for any of such reasons.
Ca. Civ. Proc. Code § 657