What is the test for a challenge for cause under section 638(1)(b) of the Criminal Code?

Williams v. The Queen (1998), 1998 CanLII 782 (SCC), 124 C.C.C. (3d) 481 (S.C.C.) was concerned with racial prejudice against native persons. The court said that challenges for cause should be permitted wherever there is a realistic potential of the existence of partiality. That realistic potential will often exist when there is widespread bias against aboriginal people in the community in question. An accused is not required to present evidence that such jurors will be unable to set aside their prejudices, as that may be an impossible task. A purposive reading of Criminal Code s. 638(1)(b) entitles the trial judge to infer, from the nature of racial prejudice itself, that some prospective jurors may be both prejudiced and unable to set that prejudice aside. The court said (at para. 22) that, "where doubts are raised, the better policy is to err on the side of caution and permit prejudices to be examined."