Evidence-Based Practice is “the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of the individual patient. It means integrating individual clinical expertise with the best available external clinical evidence from systematic research”.
Sackett, D., Rosenberg, W., Gray, J., et al. (1996). Evidence based medicine: what it is and what it isn't: it's about integrating individual clinical expertise and the best external evidence. BMJ, 312, 71-72. doi: http://dx.doi.org/10.1136/bmj.312.7023.71
There are 5 main types of clinical questions - therapy, prognosis, diagnosis, etiology/harms, and prevention. There are certain types of research studies that are best suited to answer each type of question.
After you create your PICO question, you can assess what type of question you're asking. Then, you can use the chart to determine the ideal study type to answer your question.