What is the test for determining whether a prosecutor made improper comments 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).