This [url=http://www.endangeredlanguages.com/assets/information_catalogue_endangered_languages.pdf]link[/url] on the site points to a pdf document that explains the methodology in greater detail, including some of the calculations used to come up with the level of endangerment. The authors give this caveat up front:

It is extremely important to understand that the Catalogue is a work in progress. At launch of
this website, the Catalogue is still in Phase I, which is based only on the information available in existing publications and
web resources about the individual endangered languages. Bringing in more recent and local information is critical to this
project, which is the focus of Phase II. The second phase will continue over the next two years. It involves an international
team of regional specialists (see above) reaching out to knowledgeable individuals and organizations to fill in the missing
information for languages in their areas, to check the accuracy of information, and to make needed corrections. For this
phase and long into the future, the goal is to modify, update, and improve the catalogue contents constantly, as new
information becomes available or as the situation for particular languages changes. If users of this website have particular
knowledge or information about specific languages, we encourage submission of comments and suggestions for
improvement of language entries. We are grateful for your help in improving the collective knowledge of the endangered

The Language Endangerment Index and the Need for Documentation Index presented for each language are not meant to be
the final word about degree of endangerment or extent of documentation. The scores for individual languages will change as
more information becomes available. They are provided for practical purposes, to give a quick but rough visual indication of
a language’s endangerment status and documentation needs. The level of certainty accompanying each language shows the
degree of confidence in the score: a label of “uncertain” may indicate that the level is not yet known or the score has been
computed and further evaluation is needed.