publications - Who all are considered authors of papers written on behalf of a research group?

In epidemiology it is not uncommon to see papers written by a bunch of authors on behalf of a research group (say "the X research group"). See, for example, this paper (randomly chosen from pubmed):

Moreira L, Pellisé M, Carballal S, Bessa X, Ocaña T, Serradesanferm A, Grau J, Macià F, Andreu M, Castells A, Balaguer F; on behalf of the PROCOLON research group. High prevalence of serrated polyposis syndrome in FIT-based colorectal cancer screening programmes. Gut. 2012 Sep 14.

My question is: are the members of the research group X (whose names and affiliations are usually reported in the Appendix at the end of the paper) considered as authors of the paper?

Edit: I am particularly interested to receive answers that apply specifically to epidemiology, but of course experiences from other fields are very welcome!

Answer

I'll give my perspective, as an Epidemiologist:

It depends. Helpful, I know.

Often the reason there's a "on behalf of the X group" is that some small analysis has been pulled out of a larger study, and the named authors on the paper want to acknowledge that their results are the byproduct of a larger effort. A large clinical trial, or cohort trial for example, can spawn dozens of papers and secondary analysis done by doctoral students, small component research groups, etc.

For example, this paper is (I think) the result of the "Epi proColon" clinical trial currently being conducted by the manufacturer. It's a head to head comparison with 'FIT' - fecal immunochemical testing - to detect colon cancer. The paper you linked is an ancillary analysis of that, noting a particular finding that, while of interest to the field and clearly enough for a short paper, wouldn't ever make the "main" paper cut when the results of the clinical trial are published.

@Raphael has asserted that he thinks there should be a difference between "I turned a skrew" and "I wrote the paper". The problem for Epidemiology studies is that's often somewhat ambiguous. For example, many clinical sites are directed by people who don't really care about publications, being professional clinicians, but are still instrumental enough in the conducting of the study that they could arguably be included as authors. If you only had one of these, sure, toss it in as authorship. But what if you're running a multi-site clinical trial at ten sites? Do you include all ten? After all, they saw patients.

The "group" authorship is a useful way to acknowledge that. They can cite those papers on grant applications, "why should we continue to support your diagnostic lab" progress reports, etc. It's a compromise position for trying to tighten up who is an author while at the same time supporting large, collaborative science.

Those authorship acknowledgements also serve to make something "the official position of X study group". You often see that in vaccine trials and the like.

That being said, unless I was a named author on the paper, I likely wouldn't include it on my CV if I was anything but a very new investigator. But part of the point of these papers is that who is a named author is a rotating list. The Epidemiologists write their epidemiology papers, the clinicians write clinical papers, the lab people write lab papers, the biostatisticians write...you get the idea. So everyone publishes named in their niche, but the group effort is acknowledged for the entire productive output of the study.