While commonly taught in academic settings, I disagree with the notion that it's possible to self plagiarize. It's your own words and not stealing from somebody else.
Agreed. The concept of “don’t reuse your old work when you’re supposed to be creating new work” may be valid, especially in training environments, but it shouldn’t be called self-plagiarism or treated like plagiarism.
It seems like a rule designed by journal editors to protect their turf, or PhD committees to make it easier to count original works towards degree requirements. What could possibly be the justification?
It also protects readers who may encounter the second years latter and not realize it is the same data and thus they think it is a second study reenforcing something. If they are experts in the field they likely know, but if this is a case where a different field overlaps they will want to have citations without as much knowledge of what is important.
it's really a bit of a different concept in scientific publishing, not actually plagiarism. The problematic part is publishing the same results twice, because it increases the burden on reviewers and inflates your publication count. It's also just messier if the results are in multiple places since it makes it harder to follow where those results were used and cited.