A person exposed to an infectious agent, either by contact or by vaccination, normally develops antibodies to that agent. Presumably, the severity of an infection is related to the number of antibodies produced. The degree of antibody response is indicated by saying that the person€™s blood serum has a certain titer, with higher titers indicating greater concentrations of antibodies. The following table gives the titers of twenty-two persons involved in a tularemia epidemic in Vermont (18). Eleven were quite ill; the other eleven were asymptomatic. Use an approximate t ratio to test H0: μX = μY against a one-sided H1 at the 0.05 level of significance.
The sample standard deviations for the €œSeverely Ill€ and €œAsymptomatic€ groups are 428 and 183, respectively. Severely Ill Asymptomatic Subject Titer Subject Titer

Severely III Asymptomatic Subject Titer Subject Titer 1 640 10 13 320 14 320 15 320 16 320 17 80 18 160 10 20 640 21 160 22 320 3 1280 4 160 640 6 64 7 1280 640 9 160 10 320 1 160 19