Let me begin by bursting a bubble or two...just because
it's "printed" online doesn't mean it is either accurate or correct!
Most data found online is in a pretty plain format
with no frills, although some has more information than others. As with
any form of offline research, you have to approach information obtained
online as a guide only, not as a source of primary information. Always
look for a source citation showing where the information originated.
Even if the site lists a source, the question of evaluating the quality
of the data still remains! You have to examine the information and the
source to determine whether you need to investigate further.
viewing data online which looks like it's relevant to your research,
you have to ask yourself--is the source of the information cited somewhere?
If the data is part of a project, do the volunteers list which resources
they used to transcribe the information? Take a good look at the data
and how it relates to your information--are there inconsistencies? Does
the data make sense? If there are contradictions in information, weigh
the evidence. Which piece of information seems most reasonable? Never
accept any information as valid until you can verify it, or at the very
least, have a citation for the data's source in your sweaty little hand
(my hands always get sweaty when I find new data!).