What levels of evidence are present in relation to research and practice, and why it is important regardless of the method you use?

What levels of evidence are present in relation to research and practice, and why it is important regardless of the method you use? USE this book as a reference:Melnyk, B. M. & Fineout-Overholt, E. (3rd ED). (2015). Evidence-Based Practice in Nursing & Healthcare. A Guide to Best Practice. Philadelphia: Wolters Kluwer.