Figure 1.

Relative risk of methicillin-resistant Staphylococcus aureus (MRSA) transmission by increasing quartiles of colonization pressure. MRSA colonization pressure was defined as the average proportion of total patient-days that were MRSA-positive patient-days during the preceding 7 days and was categorized into quartiles. Relative risk was calculated from log binomial regression models specifying the first quartile of colonization pressure as the reference category (broken line, RR = 1.0). *P < .05; n = number of days in each quartile; a = number of MRSA acquisitions in each quartile.