What is the test for determining whether a prosecutor made improper statements in a closing argument?

In reviewing alleged prosecutorial misconduct, we "focus[] on its asserted impropriety and substantial prejudicial effect." United States v. Weatherspoon, 410 F.3d 1142, 1145 (9th Cir. 2005). "We must . . . determine at the outset whether the prosecutor made improper statements. . . ." Id. "During closing argument, a prosecutor may do no more than comment on facts in evidence and make reasonable inferences based on the evidence." United States v. Hermanek, 289 F.3d 1076, 1101 (9th Cir. 2002).