Example: In adults suffering from depression (P), how does St. John's Wort (I) compare to SSRIs (C) in improving depression symptoms (O)?
It is important to consider the types of evidence you are looking at when researching for a PICO question. If you are conducting a thorough literature review, you want to draw from as many levels and types of evidence as possible, including both quantitative and qualitative studies, unless otherwise indicated. The Hierarchy of Evidence Pyramid is a good visual tool indicating the different types of research and their intrinsic value. Another tool that is helpful in determining level or quality of evidence is the Johns Hopkins Evidence Based Practice for Nurses and Healthcare Professionals Model.