Q: Is it compulsory to include all primary articles from previous reviews, systematic reviews, and meta-analyses when conducting a systematic review?

A systematic review is a review of existing literature answering a very specific question. So, the decision on which literature to include in your review should be based on the question you are seeking to answer.

To answer your query, you should include only primary research articles that you found in your search or that were mentioned as a reference in any of the review articles. You should not include secondary research articles. You can include unpublished or grey literature, which often yields richer, more recent insights on the topic or question you are addressing. In short, you need to include those articles that are related to your question and exclude those that are not. Otherwise, the scope of your study may become too vast. This is also critical because you will need to mention the inclusion and exclusion criteria for literature selection.

A key thing you need to keep in mind for systematic reviews and meta-analyses is that you need to register the review protocol before starting the review. You need to do this in a registry relevant to your study, such as PROSPERO. In the same registry, you will find other registered protocols, which will also help inform you whether or not your study is different from existing studies.

For more insights on secondary review articles, you may refer to the following resources:

• A young researcher's guide to a systematic review
• Secondary research – the basics of narrative reviews, systematic reviews, and meta-analysis
• Should previous systematic reviews and meta-analyses be included in a systematic review?

All the best for your review!