What is the test for "abuse of discretion" in a federal court's decision to admit or deny expert testimony?

We generally review a district courts decision to admit or deny expert testimony for abuse of discretion. United States v. Reed , 575 F.3d 900, 918 (9th Cir. 2009). However, we review de novo the "construction or interpretation of ... the Federal Rules of Evidence, including whether particular evidence falls within the scope of a given rule." United States v. Durham , 464 F.3d 976, 981 (9th Cir. 2006). Where the district court fails to engage in necessary Rule 403 balancing, we likewise review de novo . United States v.Boulware , 384 F.3d 794, 808 n.6 (9th Cir. 2004) (where "[t]he district court [ ] did not perform a Rule 403 balancing analysis," the "review [is] de novo"); see also United States v. Moran , 493 F.3d 1002, 1012 (9th Cir. 2007) (per curiam).