# Plausibility vs. probability, prior distributions, and the garden of forking paths

A discussion on plausibility vs. probability: while many given events may be plausible, but they can’t all be probable.

**By Andrew Gelman**.

I’ll start off this blog on the first work day of the new year with an important post connecting some ideas we’ve been lately talking a lot about.

Someone rolls a die four times, and he tells you he got the numbers 1, 4, 3, 6. Is this a plausible outcome? Sure. Is is probable? No. The prior probability of this event is only 1 in 1296.

The point is, lots and lots of things are plausible, but they can’t all be probable, cos total probability sums to 1.

I was thinking about this when responding to a comment about a post on a recent bit of noise from Psychological Science. Somehow we ended up discussing those classic and notorious “embodied cognition” experiments, and the exchange went like this:

Me: These sorts of studies get published, and publicized, in part because of their surprise value. (Elderly-related words can prime slow walking! Wow, what a surprise!) But when a claim is surprising, that indeed can imply that a reasonable prior will give that claim a low probability.

Daniel: With findings about priming and embodied cognition, it doesn’t seem particularly outrageous to me that a response to physical instability could easily influence our perceptions of other things in the moment as well, including romantic relationships. Part of the reason we use the peer-review process, as flawed as it is, is because experts in the field have the judgment to decide not only if a study is worthy, but if its claims are reasonably supported in the context of the theories and findings in a particular field. The hypotheses in this paper are supported by a vast amount of work on priming and embodied cognition research . . .

Martha: I was under the impression that most of the findings about priming and embodied cognition had been discredited (e.g., failure to replicate, flaws in analysis).

Daniel: Priming itself is an extremely well-replicated phenomenon. With social primes (a subset of priming research), there have been a large number of studies demonstrating the effect with a few notable failures to replicate (Doyen et al. being one of the most notable due to the online discussion that ensued). Many of the replications haven’t taken into account advances in moderators that flip around the behavioural effects of social priming research. . . .

Me: Regarding plausibility, one might equally argue, for example, that being primed with elderly-related words will make college students walk faster, as this would prime their self-identities as young people. Whatever. Theories are a dime a dozen. These papers get published because they purport to have definitive evidence, and they don’t.

Daniel: Regarding priming, I’m assuming you’re referring to Bargh’s paper and then failed replication attempt by Doyen et al. One of the most frustrating things about that exchange is that the field had already moved on to a deeper understanding of social category priming, with the Cesario, Higgins and Plaks 2006 “motivated preparation to interact account”. They found that people who like the elderly walk slower when primed, while people who dislike the elderly walk faster (among other findings). I don’t really care whether Bargh got lucky with his sample or what was going on, but a replication attempt should take into account advances in the field, particularly with a moderator that can flip the effect around.

Me: I see no strong evidence. What I see is noise-mining. Each new study finds a new interaction, meanwhile the original claimed effects disappear upon closer examination.

We could go on forever on the specifics, but here let me make a combinatorial argument. It goes like this. There could be a main effect of elderly-related words on walking speed. As discussed above, I think a main effect in either direction would be plausible. Or there could be an interaction with whether you like the elderly. Again, I can see either a positive or negative interaction making sense. There could be an interaction with sex, or socioeconomic status, or whether you have an older brother or sister. Or an interaction with your political attitudes, or with your own age, or with the age of your parents, or with whether your parents are alive, or with whether your grandparents are alive. Or an interaction with your marital status, or with your relationship status, or with whether you have kids, or with the sex of your first child. It would not be difficult to come up with 1296 of these—maybe even 1296 possibilities, each of which has appeared somewhere as a moderator in the psychology literature.

And here’s the point: any of these interactions are plausible, but they can’t all be probable. It’s not as simple as the die-rolling scenario at the top of this post—the different possible interactions are not quite mutually exclusive—but the basic mathematical idea is still there, that it’s not possible for all these large effects to be happening together.

OK, embodied cognition is a joke. No need for me to pile on it here. It’s a fading fad that eventually even Psychological Science, PPNAS, and those Ted-talk people will tire of.

Here I’m interested in something larger, which is how to think about prior distributions in this sort of open-ended research scenario.

One reason I like the idea of analyzing all comparisons at once using multilevel modeling is that this will automatically stabilize estimates. A multilevel model is a mathematical expression of the idea that these interactions can’t all be large, indeed most of them have to be small.

This is not to say that no large effects can ever exist—remember, I did say that these hypotheses are individually plausible. They’re just individually improbable, which is why we need strong evidence to move forward on them. Somebody doing a study where they found an interaction with “p less than .05,” no, that’s not strong evidence. That’s where forking paths comes in. Forking paths comes into the p-value calculation, and forking paths comes into the prior. If you want to go full Bayes, that’s fine with me, then you don’t have to worry about other analyses the researcher might have done, you just have to worry about other models of the world that are just as plausible as your current favorite (for example, “people who like the elderly walk slower when primed, while people who dislike the elderly walk faster”).

**P.S.**Commenter Garnett asked why I say that all these large effect can’t be happening together. So here’s a slight elaboration of this point:

The basic idea is that you can’t have dozens of large effects and interactions all floating around at once, as this would imply an unrealistic distribution of outcomes. Consider that notorious example of menstrual cycle and clothing, which then was said to interact with temperature and could also interact with age, relationship status, number of siblings, political and religious attitudes, etc etc etc. If many of these interactions were large, you’d end up with silly claims such as that 50% of women in some category would be wearing red on some prespecified day in their cycle.

In a predictive model, there’s only so much “juice” to go around. We’re used to thinking of this in the context of R-squared or out-of-sample prediction error, but the same principle applies when considering comparisons or effects. And, thus, once you’re considering zillions of possible effects and interactions, their common distribution has to be peaked around zero.

I think there’s a theorem in there for someone who’d like to do some digging.

**Bio: Andrew Gelman**is a professor of statistics and political science and director of the Applied Statistics Center at Columbia University. Andrew has done research on a wide range of topics, including: why it is rational to vote; why campaign polls are so variable when elections are so predictable; why redistricting is good for democracy; reversals of death sentences.

Original. Reposted with permission.

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