In The Undoing Project, Michael Lewis writes of Amos Tversky and Daniel Kahneman’s collaborative writing: 

By the time they were finished with the paper, in early 1970, they had lost any clear sense of their individual contributions. It was nearly impossible to say, of any given passage, whether more of some idea had come from Danny or from Amos. Far more easily determined, at least for Danny, was responsibility for the paper’s confident, almost brazen, tone. Danny had always been a nervous scholar. “If I had written it alone, in addition to being tentative and having a hundred references, I would probably have confessed that I am only a recently reformed idiot,” he said. “I could have done the paper all by myself. Except that if I had done it alone people would not have paid it attention. It had a star quality. And I attributed that quality to Amos.”

This is an interesting perspective to measure the outcome of teamwork, not by individual effort, specific contributions, or tangible output. 

Instead, consider what qualities the contributors imbued into the end product, without which the product would be lacking. (e.g., In editing: This person was a great co-writer who helped convey ideas clearly, that person provided the subject matter, the other person edited the piece, added star quality, and actually promoted the piece.)

In this sense, each quality is essential too. The product, and its impact, would be different without it; it would be missing a signature or a set of fingerprints.