Continuing the past discussion on authorship issues, I've been considering the questions: at what point has a person involved in the project done so little work that they should not be included as an author? For today's post, let's not consider the case of advisors, which is arguably a special case that should be considered separately (and we will return to in a further post).
It's interesting that my preferred approach of alphabetical ordering of authors arguably makes this question thornier, since the tendency of alphabetical ordering to equalize the contribution among authors suggest that there should be a contribution threshold below which you should be removed as a co-author. This is probably still true under other ordering schemes as well, but it seems more striking under alphabetical ordering.
Having thought carefully about it, I've come to a firm conclusion: it seems like a challenging issue with no right answer. In spirit, I agree with David Eppstein's comment in the previous post:
"The rule I tend to follow is that there are no 1% coauthors. If you haven't contributed an idea that you can point to as part of the paper, or done some of the hard work of turning ideas into proofs, or done some of the implementation or data analysis, you don't get to be a co-author."
In practice, I've found it's a lot fuzzier. Sometimes the "idea" is the initial formulation of the problem, not the solution. Should that count as a contributed idea? Many papers, I find, boil down to one key idea, and the rest is just details. Four of you spend a bunch of time kicking around ideas that don't work, then one sees the idea that works, and the paper really writes itself from there. Should the other three not be co-authors? These aren't hypotheticals. I've been in situations where I've talked with someone about a problem, which I considered at that point "my" problem, with the idea that we would possibly work together on it, only to have them present me with a finished paper they were submitting on their own a few months later (apparently without realizing that I might be peeved by this). I've been in a situation where one co-author, after much dead-end work by everyone, came up with the key insight and then thought that the result should be a single-author paper. I'm sure I (and others) can come up with similarly unclear situations.
I recognize that, in both of these circumstances, one could argue that the other party was in the right. But it really doesn't matter who was right; in the end, these became people I would be very wary of working with in the future (and perhaps they felt the same of me). As we've discussed before, one of the great benefits of collaborating is establishing long-term collaborations over multiple problems. That didn't happen. What I've learned was that, in such situations, one should be clear ahead of time what the expectations are. Being clear when starting a collaboration about expectations might be a little awkward, but seems better than awkwardness later. If you're telling someone a problem and consider it from that point on a collaboration, tell them. I personally think the default assumption should be that if you're in the room on multiple occasions when working on a problem, you should be considered a co-author even if you're not the one to make the breakthrough, but if there's a reason that shouldn't be the case, somebody should speak up sooner rather than later. Different people may use different rules or guidelines for what they consider sufficient for co-authorship on a paper, and setting expectations is really the key.
As another story, I should admit I have been involved in a paper where I was a 1% contributor. I started working with a group on a problem, but it was a group in a distant location, and while I was there -- for a short period -- no progress was made. The group, with much more work, eventually solved the problem. I was asked by the senior author whether I wanted to be on the paper, and I declined. I thought a potentially awkward situation was handled perfectly by this senior author. They were up front that, as it turned out, I hadn't contributed tangibly to the paper, but were willing to accept that, in this case, it was an accident of circumstance rather than intent. I was glad they respected me enough to discuss it openly and leave it as my decision, even though in my mind the proper course was clear. (Indeed, in this case, I thought it was so clear I would not have been surprised or offended if they hadn't offered.) I assume that if I had decided otherwise I would have been a co-author on the paper without argument, and they would then have to consider that history in future dealings with me. I also think that, if I had said I wanted to be a co-author on the paper, it would have been appropriate for them to add me, because perhaps we had had different perceptions and expectations regarding the work.
In any case, I think the issue is a difficult one, made even more so because it's tied to quite directly to one's job performance, which tends to color self-perception. Perhaps others can add more clarity to the question in the comments.