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On the science & art of list definition:
The science: To be listed, it is necessary for a feed to have at least 1,000 Twitter followers, at least two of whom are list members, and for the feed to belong to an individual, not to a collective (e.g., department or publication).
The rest is art: Generally, a Ph.D. in a field associated with the list is required. Note that there is a separate list for astronomers and related professionals. Exceptions to the Ph.D. rule are made in some cases for those who are Ph.D. students, who have a following for their outreach efforts, and/or who are followed by many other list members. Users who meet the Ph.D. rule but who do not work professionally in the field of the list may be included if they tweet reasonably frequently on topics pertinent to the list field.