Patient population (or problem to be addressed):
Comparison (Control, Placebo, Standard Care)
Note that some version of PICO add one or two "T"'s to the end. These may cover such things as:
Defining your inclusion and exclusion criteria in advance is just as important in a systematic review as it is in a clinical study.
These are often called “Limits” during search process and are determined by research question.
These criteria determine which ‘subjects’ (studies) will be included in or excluded from the review.
Inclusion/exclusion criteria usually include standard things such as choice of languages, publication dates, publication types, geographical locations, but - more importantly - they may also include your outcomes measures. This is why pre-determining your outcome measures when formatting your question is so useful. If you are looking for studies that report the results of a certain test, any study that doesn't report such results is then excluded from your review.
Also, think about your population carefully, just as you would for a clinical trial. Not just age groups and genders, but also comorbidities and related conditions.