Each paper should be able to provide enough evidence that its claims are correct. Nowadays, where the number of joint industry-university projects grows exponentially, many confidential documents produced by private companies become available to researches in academia under Non Disclosure Agreements (NDA).
My question is the following: is it ethical to cite a confidential work in a scientific paper and use it to make some statements, while the rest of the scientific community cannot evaluate the accuracy of the claims in that source?
In my personal opinion, this type of references should be prohibited, since they hinder the progress of science, but I would be glad to hear other opinions on this topic and especially how they believe that such a case should be treated.
What's the alternative? To not cite the source, and fail to credit it when you've built on it? To not publish your own paper since you've built on confidential work, thereby hindering the progress of science? (Assuming your work would have furthered the progress of science, of course.)
When you cite confidential sources with enough information, other researchers can at least ask the source about the claims you have made based on it. This seems better (ethically) than any of the alternatives mentioned above.
If there's also a non-confidential source that supports your claim you can cite that, but that isn't always an option.