How much can you trust a reference of a paper that is not available It recently occurred to me that probably more often than it should happen, while reading someones paper, they see a reference to a different paper, and just use the reference without looking.  In my area, a common example is in referencing data, such as average human height, etc.

I came across this as I was looking for data on human dimensions from a somewhat smaller country that does not use English.  I found 9 papers that all referenced the same data sheet.  I then contacted the author of that data sheet and was informed that there is no online version (never has been), and the only print copy exists in that country.  The author herself does not have this book.  I'm guessing the only reason the other authors referenced the data was because the author of the referenced data wrote a paper that referenced her own data.

I am not interested in how to format these citations. I would like to know if it is appropriate to cite this information at all, or cite that someone else cited it.  I am curious as I am not sure about the accuracy of the information.  If I do cite someone else's citation (which i am 99% confident they never saw the data), it seems like I can just make something up myself.  Is there a way to express that maybe this data is accurate, but that I'm not sure, since it is the only available source about that data?  Or should I ignore it all together and pretend as if there is no data?
Is there a way to express this (not as a citation format), that maybe this data is accurate, but not sure, since it is the only available source about that data.


How about something like: "X, measured by Jones in 1950, is commonly given as the value for Y (as cited in Smith, 1989 and Cutler, 1995). However, the original manuscript by Jones describing this result is not generally available."


  Or should I ignore it all together and pretend as if there is no data.


Don't do this. If X is cited often in the field, then by pretending as if there is no data, you give the appearance of not knowing your field. 

Whether or not you should use X (knowing that it is unreliable) depends on your purposes.

If your work hinges on having an accurate value for X, then you definitely should not use X if you cannot track down a reliable source for it. (Nor should you make something up.)

If you are using X just as a "sane" value for something, and it isn't central to your work, on the other hand:

If you think you have a better value than X, and can justify why you are using a different value than everyone else in your field, go ahead.

Otherwise, if you need a value, you should just use X (unreliable as it is). A bit of unreliable data that is accepted in your field is still better than a bit of unreliable data that you just made up with no justification.