What is the test for peremptory challenges in a murder trial?

The right is particularly critical for one accused of a capital offense. The jury must decide not only guilt, but life itself. A jury must "express the conscience of the community on the ultimate question of life or death." (Witherspoon v. Illinois (1968) 391 U.S. 510, 519, 88 S.Ct. 1770, 1775, 20 L.Ed.2d 776.) Any restriction on the exercise of peremptory challenges in this context threatens the fairness of the jury not only as to guilt but also as to penalty. ¹¹