H Imogen French!
When I write a hub I always cite a reference source for any factual assertion I make, or any information I don't believe is common knowledge (books and articles, and a few online sources). I do this so that readers can feel free to do more reading on the topics and see if I've misrepresentated the sources in anyway.
When it comes to statistics, of course, that is very tricky. Different, perfectly reputable sources give different statistics on various areas of data. Statistics are "fuzzy" by their nature. People should not make arguments that are ruthlessly dependent upon statistics for this reason.
Of course, if the author says something like: "... Cairo, the capital of Spain," then one should say something in good conscience.
Take it easy.