I'm a PhD student in statistics. I use a lab notebook (well, Emacs org-mode file) for my work, much of which involves writing software to implement my research

I'm a PhD student in statistics. I use a lab notebook (well, Emacs org-mode file) for my work, much of which involves writing software to implement my research ideas. The notebook is useful in quite a few ways. Often in a project you investigate many different ideas (should I implement the algorithm this way? a different way?) and discard those that don't work, and the notebook is a record of the reasons I threw away each idea. When the idea pops up six months later, or I need to mention it in a paper, I can remember what the results were and why it failed. When I'm investigating a weird bug or exploring some possible improvement, the notebook records my progress. I might be profiling a slow chunk of code to find potential improvements, pasting each profile into the journal, and then refer back to them to see if my changes have helped. I also record various ideas I have but don't have time to explore, papers I've read with potentially useful information, research progress, and all sorts of useful stuff. I find I don't actually have to refer back to old entries very often -- mostly just to the last couple days as I'm working through a problem -- but it's very reassuring to have around. I suspect the role of the lab notebook is subsumed by public bug trackers and mailing lists for collaborative developers -- if you're tracking down a bug, you might post your progress in the GitHub or Bugzilla issue so your collaborators can follow along. I know Mozilla carefully links commits to bugs so they have a permanent record of every change that went into their code.