An important characteristic of a systematic review is that the search is replicable: someone else should be able to follow the strategy and end up with the same (or very similar) articles.
It’s very easy to look at an article and decide that you want to include it in your study, because the results look good or you’ve heard of the authors, or any other factors. To eliminate these personal biases (as far as possible), it’s crucial to develop clear inclusion and exclusion criteria before beginning your final search.
To do this, you need to consider exactly what you need to find out, and where you might find it. Some examples of criteria you might need to consider include:
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There are many other criteria that might also need to be considered.