A dissertation or thesis is a work that supports a doctorate or master’s degree
The paper presents the author's research and findings. Many databases contain dissertations and theses, including full-text. Electronic versions of theses and dissertations are called ETDs which has made organizing and distributing the documents much easier. Many databases require a paid subscription. But recently these resources are being made available through publishers including university presses (academic publishers).
Previously published journal articles as dissertation chapters
The practice of dissertation authors including their own previously published journal articles as chapters in their official submissions goes by several names: composite thesis, sandwich thesis, and thesis-by-publication are a few of the most common constructions. Whatever one calls it, the practice is overwhelmingly found in the STEM disciplines, where students tend to publish coauthored papers with their lab groups prior to graduation. A different, but related, practice is the “manuscript” option, in which a group of publication-ready papers form the dissertation, with the expectation that students will submit them to publishers after graduation.
Why consult dissertations?
Because the researcher has gone to great lengths to research the topics. You can gain from all of their hard work combing the bibliography for primary and secondary sources. You can also study how they approached the topic. What type of method did they use? Did they include a statistical analysis? How persuasive is their argument? Did you find the writing engaging? If so, can you learn from this example to improve your own writing?
