Such views are essential to putting the story together. Papers delivered at conferences and published as conference proceedings can be used in literature reviews. Usually, but not always, those papers have passed through a review process that attests to their academic merit. Use their conclusions with caution. Students should check with their advisor to see if trade magazines are acceptable in their literature review.
Newspapers can be a source of unsubstantiated information.
One should be careful in using them. Written material collected from companies or organizations sometimes are appropriate for use in literature reviews. Such information includes annual statement or internally produced materials.