The posttrauma heart rate can be either greater than or less than the healthy rate; thus, a two-sided test is used. z1-~/2 = 1.96, and Z l-~ = 0.84. From Eq. (22.2),

)2 2/d2 (1.96 + 0.84)2(9.1)2/62 18.0. nl = n2 -- (z l-a/2 -+- z 1-/3 cr -- --

The physician needs at least 18 patients. However, for reasons set forth in Section 22.2, he should choose a few more than 18.