A study is conducted comparing two antibiotics for the treatment of UTIs. The results after 7 days of therapy demonstrated that Drug A was superior to Drug B in eradicating bacteria from the urine and eliminating signs and symptoms of infections. The investigators concluded that Drug A was significantly superior to Drug B (p<0.05). In subsequent studies, however, the two drugs were equivalent in efficacy.
Which one of the following types of error apparently occurred in the first study?