Two things are necessary in order to do a systematic review:

1. The research question

A well defined research question is absolutely crucial for the relevance and success of a systematic review. Almost every other decision that you will have to make, while working with the review, will depend on how you define and limit the question it aims to answer, or map the available evidence for.

PICO is an often used frame for systematic review research questions. It was invented for questions in evidence-based medicine, but with a little adaptation it can be used in many other subjects as well.

P = Population
I = Intervention
C = Comparison
O = Outcome

Not every question can be translated into the PICO categories. The important thing is that the question and its elements must be clearly stated, in order to develop a structured and exhaustive search strategy and useful selection criteria. Two rather colourful other designs are shown below. The general idea with all these acronyms is however the same: by grouping your search terms in blocks, that represent the mandatory elements that must be included in a text for it to be interesting for your research question, you gain control of how for example adding or deleting terms affects the result of the search.

2. The inclusion and exclusion criteria

The very comprehensive search that you need, to be certain that nothing important is missed, means that there will be a big and sometimes tedious work screening the search results to find the really relevant articles. In order to work with this in a structured and reproducible way, it is recommended to define, in advance, the criteria for which publications should be selected. Inclusion criteria are also a possibility to agree on how the research question or search terms should be interpreted in the study. And finally, not every aspect of a question can be captured with a search strategy without risking the comprehensiveness. The selection criteria are an important part of the search strategy.

The following is an example of the PICO-framed question and the selection criteria for a published study with the title: “Corn yield response to winter cover crops: An updated meta-analysis”

P: Crops of Zea Mays
I : Cover Crop Treatment
C: No CCT (comparison with other year or other location)
O: Yields

Selection criteria:

Yield records came from corn following a cover crop treatment, and corn following no cover (NC)
Yields were reported in more than one year or location
Enough information was provided to compute study variances
The studies were conducted in the United States or Canada

Marcillo, GS and Miguez, FE (2017). "Corn yield response to winter cover crops: An updated meta-analysis." Journal of Soil and Water Conservation 72(3): 226-239.