The minimum requirement for authorship is that an author must have had a substantial intellectual contribution to the paper, where one of the following conditions is met:
a) conception and design; and/or
b) analysis and interpretation of data; and/or
c) drafting the article or revising it critically so as to contribute to the interpretation
Researchers planning to write a paper, who have benefited from a statistical consultation, should consider whether the statistician has met one of the above requirements. Examples of ways they could meet these criteria include at least one of the following:
They made substantial contributions in the development of the design or analysis plan
They recommended significant changes to a previous design or analysis plan that were followed.
They provided significant material support in analysis, e.g. writing example R code, or actually undertaking parts of the analysis.
As for any collaborator, whether or not the statistician was paid for their services should not be a consideration when assessing authorship, nor should the length of time they dedicated to the problem. The key point is whether there was a substantial intellectual contribution to design, analysis or interpretation. Providing feedback on pre-existing plans, or providing general suggestions without significant material support, would typically not warrant co-authorship, but should be acknowledged in publications (pending approval from the statistician).
The Vancouver protocol on co-authorship further states that a co-author should satisfy the following criteria:
Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
Drafting the work or revising it critically for important intellectual content; AND
Final approval of the version to be published; AND
Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved
Thus, in cases where co-authorship is warranted, the consultant (and all co-authors) should be consulted in the write-up process, and should give final approval prior to submission.
If you have any questions or feedback about this policy, feel free to discuss with your consultant or the Stats Central Director (firstname.lastname@example.org)