A valid piece of research truly measures what it sets out to measure.
To give valid results, a piece of research needs to:
These questions can be asked of all levels of evidence and all types of questions, and are equally important for systematic reviews or guidelines.
Conclusions in the abstract and discussion may be biased by the author’s pre-determined ideas about the results.
You should ensure that the results and the conclusions are concordant. Have the authors discussed other work that both supports and contradicts their findings or have they focused on one aspect only?