Final answer:
Administering ampicillin before surgery decreases the risk of staphylococcal infection.
Step-by-step explanation:
Based on the data in Table 15.1, the effect of administering ampicillin before surgery on the risk of staphylococcal infection is that it decreases the risk of infection. The relative risk of acquiring a hospital-acquired infection (HAI) is less than one when the patient is given a pre-surgery antibiotic, indicating a reduced risk. In fact, taking ampicillin before surgery cuts the risk of HAI by almost half.