What is the legal test for a claim of ineffective assistance of counsel?

"Reviewing courts defer to counsel's reasonable tactical decisions in examining a claim of ineffective assistance of counsel [citation], and there is a 'strong presumption that counsel's conduct falls within the wide range of reasonable professional assistance.' [Citation.] Defendant's burden is difficult to carry on direct appeal, as we have observed: ' "Reviewing courts will reverse convictions [on direct appeal] on the ground of inadequate counsel only if the record on appeal affirmatively discloses that counsel had no rational tactical purpose for [his or her] act or omission." ' [Citation.]" (People v. Lucas (1995) 12 Cal.4th 415, 436-437, criticized on other grounds in People v. Bolin (1998) 18 Cal.4th 297, 315.)