PICO or PICOT is an acronym for a series of important concepts in formulating an evidence based research question.
PICOT stands for:
P: Patient, Population, or Problem
C: Comparison or Control
T: Time period (not always included)
When choosing studies to back up your conclusion, you want the best evidence available. What studies are best? This is the "Coding Matrix," the levels of strength, used in the Evidence-Based Care Sheets in CINAHL.
A meta-analysis is the strongest evidence, and conference papers are the least strong. Use the strongest evidence types you can find--but remember that there aren't meta-analyses and randomized controlled trials for everything! Sometimes you have to use weaker research, because that's all that's been done, so far.