When I was a Ph.D. student, it was often confusing to me who should I put as the "second author" vs. "third author", and the like. Of course, as a young kid, I want to make everyone happy. Fortunately, I was involved in an ethics campaign organized by Japan Research Society where I was asked to learn the "right" way of authorship crediting. Here I would like to summarize some key points:
What is considered "significant contribution"
- Drafting and revising the papers, laying out the logical flow of the main concept
- Conducting analysis that requires certain technical expertise (e.g., machine learning, neural imaging analysis)
- Providing important ideas to the experiment design and executing the experiments
- What is considered "insignificant contribution"
- Conducting "way too simple" analysis (e.g., finding an average, making beautiful graphs; put them in Acknowledgement though)
- Providing funding, finding participants, etc. (i.e., put them in Acknowledgement as well)
- Checking grammar
Nature has recently published a survey, revealing the opinions of 6000 researchers, regarding who deserves authorship. Take a look.