We are pleased to announce that the Proceedings has begun accepting Supplementary Material that will appear only in the online version of the journal. This new feature will expand the possibilities for publishing in the Proceedings without increasing the size of the printed journal. Supplementary Material is reserved for information directly relevant to the conclusions of the paper but not included because of space or format restrictions. This material will be reviewed along with the rest of the paper.
What information should be supplementary and what should appear in the body of the paper? Supplementary Material should be important to the paper, but not essential. It should supplement a paper that can stand alone without it. It might be the extra figure or table that buttresses the conclusions of the paper but had to be removed to make the paper fit within the character limit. It might be the much abused “data not shown” which would allow the reader and reviewer to no longer have to take the author’s word on an important point.
Mathematical proofs and derivations are often printed only in outline form because of their length and technical nature. Now the highlights and implications of this type of work can appear in print and the formal demonstration can appear online as Supplementary Material for those who wish to follow the arguments in detail.
Some Supplementary Material, such as audio and video clips, lends itself only to an online format. We hope that authors will use this opportunity in an imaginative way. Very large data sets developed in areas from genomics to cosmology might also be suitable for Supplementary Material.
We are aware of the risk of having two classes of information, both in the body of the paper and in the supplement. To some degree such a dichotomy is already upon us and is growing as the unique possibilities of electronic publishing are realized. We ask authors to use Supplementary Material responsibly. Although online supplements fell no trees, they do use space and add expense. Supplementary Material should not become a burden for reviewers or a data dump. Reviewers and editors will be the gatekeepers of content.
Supplementary Material will be available online and free to everyone—a subscription is not necessary. More details are given in the Information for Authors on page xvi, which appears at the back of each issue and on our web site at www.pnas.org.