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"Each dissertation published since July, 1980 includes a 350-word abstract written by the author. Master's theses published since 1988 include 150-word abstracts. Simple bibliographic citations are available for dissertations dating from 1637. Where available, PQDTGlobal provides 24-page previews of dissertations and theses."
For abstracts from 1938-1980 see the print volumes of Dissertation Abstracts International
"ProQuest Dissertations and Theses: Global (PQDTGlobal) is the world's most comprehensive collection of full-text dissertations and theses. As the official digital dissertations archive for the Library of Congress and as the database of record for graduate research, PQDTGlobal includes millions of searchable citations to dissertations and theses from 1861 to the present day together with over a million full-text dissertations that are available for download in PDF format. Over 2.1 million titles are available for purchase as printed copies. The database offers full text for most of the dissertations added since 1997 and strong retrospective full-text coverage for older graduate works. It also includes PQDT UK & Ireland content."