One constant refrain about the kids these days is that they use way too much supplementary material. All those important controls, buried in the supplement! All the alternative hypotheses that can’t be ruled out, buried in the supplement! All the “shady data” that doesn’t look so nice, buried in the supplement! Now papers are just reduced to ads for the real work, which is… buried in the supplement! The answer to the ultimate question of life, the universe and everything? Supplementary figure 42!
Whatever. Overall, I think the idea of supplementary figures makes sense. Papers have more data and analyses in them than before, and supplementary figures are a good way to keep important but potentially distracting details out of the way. To the extent that papers serve as narratives for our work as well as documentation of it, then it’s important to keep that narrative as focused as possible. Typically, if you know the field well enough to know that a particular control is important, then you likely have an interest sufficient enough to go to the trouble to dig it up in the supplement. If the purpose of the paper is to reach people outside of your niche–which most papers in journals with big supplements are attempting to do–then there’s no point in having all those details front and center.
(As an extended aside/supplementary discussion (haha!), the strategy we’ve mostly adopted (from Jeff Gore, who showed me this strategy when we were postdocs together) is to use supplementary figures like footnotes, like “We found that protein X bound to protein Y half the time. We found this was not due to the particular cross-linking method we used (Supp. Fig. 34)”. Then the supplementary figure legend can have an extended discussion of the point in question, no supplementary text required. This is possible because unlike regular figure legends, you can have interpretation in the legend itself, or at least the journal doesn’t care enough to look.)
I think the distinction between the narrative and documentary role of a paper is where things may start to change with the increased focus on reproducibility. Some supplementary figures are really important to the narrative, like a graph detailing an important control. But many supplementary figures are more like data dumps, like “here’s the same effect in the other 20 genes we analyzed”. Or showing the same analysis but on replicate data. Another type of supplementary figure has various analyses done on the data that may be interesting, but not relevant to the main points of the paper. If not just the data but also the analysis and figures are available in a repository associated with the paper, then is there any need for these sorts of supplementary figures?
Let’s make this more concrete. Let’s say you put up your paper in a repository on github or the equivalent. The way we’ve been doing this lately is to have all processed data (like spot counts or FPKM) in one folder, all scripts in another, and when you run the scripts, it takes the processed data, analyzes it, and puts all the outputted graphical elements into a third folder (with subfolders as appropriate). (We also have a “Figures” folder where we assemble the figures from the graphical elements in Illustrator; more in another post.) Let’s say that we have a side point about the relative spatial positions of transcriptional loci for all the different genes we examined in a couple different datasets; e.g., Supp Figs. 16 and 21 of this paper. As is, the supplementary figures are a bit hard to parse because there’s so much data, and the point is relatively peripheral. What if instead we just pointed to the appropriate set of analyses in the “graphs” folder? And in that folder, it could have a large number of other analyses that we did that didn’t even make the cut for the supplement. I think this is more useful than the supplement as normally presented and more useful than just the raw data, because it also contains additional analyses that may be of interest–and my guess is that these analyses are actually far more valuable than the raw data in many cases. For example, Supp Fig. 11 of that same paper shows an image with our cell-cycle determination procedure, but we had way more quantitative data that we just didn’t show because the supplement was already getting insane. Those analyses would be great candidates for a family of graphs in a repository. Of course, all of this requires these analyses being well-documented and browsable, but again, not sure that’s any worse than the way things are now.
Now, I’m not saying that all supplementary figures would be unnecessary. Some contain important controls and specific points that you want to highlight, e.g., Supp. Fig. 7–just like an important footnote. But analyses of data dumps, replicates, side points and the such might be far more efficiently and usefully kept in a repository.
One potential issue with this scheme is hosting and versioning. Most supplementary information is currently hosted by journals. In this repository-based future, it’s up to Bitbucket or Github to stick around, and the authors are free to modify and remove the repository if they wish. Oh well, nothing’s permanent in this world anyway, so I’m not so worried about that personally. I suppose you could zip up the whole thing and upload it as a supplementary file, although most supplementary information has size restrictions. Not sure about the solution to that.
Part of the reason I’ve been thinking about this lately is because Cell Press has this very annoying policy that you can’t have more supplementary figures than main figures. This wreaked havoc with our “footnote” style we originally used in Olivia’s paper because now you have to basically agglomerate smaller, more focused supplementary figures into huge supplementary mega-figures that are basically a hot figure mess. I find this particularly ironic considering that Cell’s focus on “complete stories” is probably partially to blame for the proliferation of supplementary information in our field. I get that the idea is to reduce the amount of supplementary information, but I don’t think the policy accomplishes this goal and only serves to complicate things. Cell Press, please reconsider!