What is the test for evidence under Rule 404(b)?

1 Although we stated in United States v. Herrera-Medina, 609 F.2d 376 (9th Cir. 1979), that the prior criminal conduct offered into evidence must be similar to the offense charged, this prerequisite obviously does not apply to all evidence offered under Rule 404(b). If the purpose of the proffered evidence is to show that the defendant's prior criminal conduct provided him with opportunity, knowledge, preparation or motive, it may or may not be necessary that the prior conduct be similar to the offense charged. But when evidence is offered to prove identity, modus operandi, or absence of mistake or accident, the prior criminal conduct is relevant only if it is similar to the offense charged.