Searching in Nursing Reference Center Plus for the topic "AACN levels of evidence" will take you to a good resource on choosing the best levels of evidence for your clinical question if using the AACN model.
Peterson, M.H. Barnason, Sl., Donnelly, B., Hill, K. Miley, H., Riggs, L., & Whitman, K. (2014). Choosing the best evidence to guide clinical practice: Application of AACN levels of evidence. Critical Care Nurse, 34(2), 56-68.