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Seminal Sources
Identifying landmark/seminal studies is a process, a part of the research process, and relies on your thoroughness as a researcher. Typically landmark/ seminal studies are not labeled as such but can be identified through continued reading and comparing of articles. Below are a few ways to find the elements of your topic.
"In terms of research, a seminal source refers to a key researcher, research study, model, or theory. Here potent means original and deeply influential. An example of a seminal researcher is Edward Deci. Few people know his name, but the term intrinsic motivation might be familiar. Deci was one of the first and most influential researchers to experimentally explore the construct of intrinsic motivation (Deci & Ryan, 2000). Clearly reading his work may help a researcher better understand the notion of intrinsic motivation."
From Greenberg, S. (2015). Foundations of empirical research. In Grand Canyon University (Ed.), GCU doctoral research: foundations & theories. https://lc.gcumedia.com/res850/gcu-doctoral-research-foundations-and-theories/v1.1/#/chapter/2
The best way to recognize a study is to look at the bibliography of several articles on the same subject. If the same article (researcher or study) is cited in most or all of the articles, it is very likely that you have found a landmark study.
Another way to check to see if the article you have found is a landmark study is to do a Cited Reference Search in Web of Science, or Google Scholar. Landmark studies will be cited by hundreds of other articles.
It is important to note that seminal or landmark studies have been published many years ago and if you are limiting your search by date you may miss the original study in your results. You may still find references to the original landmark study in newer articles.
NOTE: although all seminal articles will be highly cited, not all highly cited articles are necessarily seminal. In fact, the number of citations an article has received is not necessarily an indication of the article's quality. An article may have been mentioned in many other articles because it was particularly problematic, for example; other authors may have pointed out problems with its methodology, conclusions, etc.