Efficacy of echinacea, duration of symptoms. The study introduced in the prior exercise also compared duration of peak symptoms in the treatment and control group. The treatment group (n1 = 337) had a mean duration of 1.60 days (standard deviation 0.98 days). The control group (n2 = 370) had a mean duration of peak symptoms of 1.64 days (standard deviation 1.14 days). Test the difference in means for significance.