What is the test for a motion for a new trial based on newly discovered evidence?

5 Rule 33 allows a district court upon motion by a defendant, to grant a new trial "if required in the interest of justice." A motion for a new trial based on newly discovered evidence, however, will only be granted when admission of the newly discovered evidence "would probably lead to an acquittal." United States v. Alessi, 638 F.2d 466, 479 (2d Cir.1980) (citations omitted).