After writing my last post critiquing Karl Friston’s commentary in NeuroImage, I emailed him the link, figuring he might want the opportunity to respond, and also to make sure he knew my commentary wasn’t intended as a personal attack (I have enormous respect for his seminal contributions to the field of neuroimaging). Here’s his very classy reply (posted with permission):
Many thanks for your kind e-mail and link to your blog. I thought your review and deconstruction of the issues were excellent and I concur with the points that you make.
You are absolutely right that I ignored the use of high (corrected) thresholds when controlling for multiple comparisons – and was focusing on the simple case of a single test. I also agree that, ideally, one would report confidence intervals on effect sizes – indeed the original version of my article concluded with this recommendation (now the last line of appendix 1). I remember – at the inception of SPM – discussing with Andrew Holmes the reporting of confidence intervals using statistical maps – however, the closest we ever got was posterior probability maps (PPM), many years later.
My agenda was probably a bit simpler than you might have supposed – it was to point out that significant p-values from small sample studies are valid and will – on average – detect effects whose sizes are bigger than the equivalent effects with larger sample sizes. I did not mean to imply that large studies are useless – although I do believe that unqualified reports of significant p-values from large sample sizes should be treated with caution. Although my agenda was fairly simple, the issues raised may well require more serious consideration – of the sort that you have offered. I submitted the article as a ‘comments and controversy’, anticipating that it would elicit a thoughtful response of the sort in your blog. If you have not done so already; you could prepare your blog for peer-reviewed submission – perhaps as a response to the ‘comments and controversy’ at NeuroImage?
I will not respond to your blog directly; largely because I have never blogged before and prefer to restrict myself to peer-reviewed formats. However, please feel free to use this e-mail in any way you see fit.
With very best wishes,
PS: although you may have difficulty believing it – all the critiques I caricatured I have actually seen in one form or another – even the retinotopic mapping critique!
Seeing as my optimistic thought when I sent Friston the link was “I hope he doesn’t eat me alive” (not because he has that kind of reputation, but because, frankly, if someone obnoxiously sent me a link to an abrasive article criticizing my work at length, I might not be very happy either), I was very happy to read that. I wrote back:
Thanks very much for your gracious reply–especially since the tone of my commentary was probably a bit abrasive. If I’m being honest with myself, I’m pretty sure I’d have a hard time setting my ego aside long enough to respond this constructively if someone criticized me like this (no matter how I felt about the substantive issues), so it’s very much appreciated.
I won’t take up any of the substantive issues here, since it sounds like we’re in reasonable agreement on most of the issues. As far as submitting a formal response to NeuroImage, I’d normally be happy to do that, but I’m currently boycotting Elsevier journals as part of the Cost of Knowledge campaign, and feel pretty strongly about that, so I won’t be submitting anything to NeuroImage for the foreseeable future. This isn’t meant as an indictment of the NeuroImage editorial board or staff in any way; it’s strictly out of frustration at Elsevier’s policies and recent actions.
Also, while I like the comments and controversies format at NeuroImage a lot, there’s no question that the process is considerably slower than what online communication affords. The reality is that by the time my comment ever came out (probably in a much abridged form), much of the community will have moved on and lost interest in the issue, and I’ve found in the past that the kind of interactive and rapid engagement it’s possible to get online is very hard to approximate in a print forum. But I can completely understand your hesitation to respond this way; it could quickly become unmanageable. For what it’s worth, I don’t really think blogs are the right medium for this kind of thing in the long term anyway, but until we get publisher-independent evaluation platforms that centralize the debate in one place (which I’m hopeful will happen relatively soon), I think they play a useful role.
Anyway, whatever your opinion of the original commentary and/or my post, I think Friston deserves a lot of credit for his response, which, I’ll just reiterate again, is much more civil and tactful than mine would probably have been in his situation. I can’t think of many cases either in print or online when someone has responded so constructively to criticism.
One other thing I forgot to mention in my reply to Friston, but is worth bringing up here: I think SPM confidence interval maps would be a great idea! It would be fantastic if fMRI analysis packages by default produced 3 effect size maps for every analysis–respectively giving the observed, lower bound, and upper bound estimates of effect size at every voxel. This would naturally discourage researchers from making excessively strong claims (since one imagines almost everyone would at least glance at the lower-bound map) while providing reviewers a very easy way to frame concerns about power and sample size (“can the authors please present the confidence interval maps in the appendix?”). Anyone want to write an SPM plug-in to do this?