Issues on reference table

[SteveViss]

• Some ids (jstor, pmid, doi) contain NA instead of null.
• jstor field is inconsistent (some have url, some just the id)

[tpoisot]

some years contains NA instead of null, and some years have letters at the end

Example: http://poisotlab.biol.umontreal.ca/api/v2/reference?year=1976a

[SteveViss]

Note: Field year should be declared as integer in SQL table to avoid both problems