Problem: Please examine the hierarchy of the significance of evidence, particularly about my evaluation of existing literature. Research topic: In adult inpatients in critical care and step-down units (P), how does a multimodal infection prevention protocol (I) compared to standard precautions, including aseptic technique for device insertion and maintenance (C) affect rates of hospital-acquired infection (O) within six months (T)? Which level of evidence is the most elusive to locate? Why do you think this is?