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IlyaShpitser comments on LINK: AI Researcher Yann LeCun on AI function - Less Wrong
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Comments (82)
Sure. Here's the world's simplest causal graph: A -> B.
Rubin et al, who do not like graphs, will instead talk about a joint distribution:
p(A, B(a=1), B(a=0))
where B(a=1) means 'random variable B under intervention do(a=1)'. Assume binary A for simplicity here.
A causal model over A,B is a set of densities { p(A, B(a=1), B(a=0) | [ some property ] } The causal model for this graph would be:
{ p(A, B(a=1), B(a=0) | B(a=1) is independent of A, and B(a=0) is independent of A }
These assumptions are called 'ignorability assumptions' in the literature, and they correspond to the absence of confounding between A and B. Note that it took counterfactuals to define what 'absence of confounding' means.
A regular Bayesian network model for this graph is just the set of densities over A and B (since this graph has no d-separation statements). That is, it is the set { p(A,B) | [no assumptions] }. This is a 'statistical model,' because it is a set of regular old joint densities, with no mention of counterfactuals or interventions anywhere.
The same graph can correspond to very different things, you have to specify.
You could also have assumptions corresponding to "missing graph edges." For example, in the instrumental variable graph:
Z -> A -> B, with A <- U -> B, where we do not see U, we would have an assumption that states that B(a,z) = B(a,z') for all a,z,z'.
Please don't say "Bayesian model" when you mean "Bayesian network." People really should say "belief networks" or "statistical DAG models" to avoid confusion.
I do not mean "Bayesian networks". I mean Bayesian models of the kind e.g. described in Gelman's Bayesian Data Analysis.
You still can express this as plain-vanilla conditional densities, can't you? "under intervention do(a=1)" is just a different way of saying "conditional on A=1", no?
and
I don't see counterfactuals in your set of densities and how "interventions" are different from conditionality?
No. If conditioning was the same as interventions I could make it rain by watering my lawn and become a world class athlete by putting on a gold medal.
I don't understand -- can you unroll?
Well, since p(rain | grass wet) is high, it seems making the grass wet via a garden hose will make rain more likely. Of course you might say that "making the grass wet" and "seeing the grass wet" is not the same thing, in which case I agree!
The fact that these are not the same thing is why people say conditioning and interventions are not the same thing.
You can of course say that you can still use the language of conditional probability to talk about "doing events" vs "seeing events." But then you are just reinventing interventions (as will become apparent if you try to figure out axioms for your notation).
That's a strawman. The conditional probability we're talking about has a clear (if explicitly unstated) temporal ordering: P(rain in the past | wet grass in the present).
Talking about conditional probability was widespread long before people started talking about interventions.
It seems to me that the language of interventions, etc. is just a formalism that is convenient for certain types of analysis, but I'm not seeing that it means anything new.
You seem to be missing Ilya's point. He was arguing that if you regard "under intervention do(A = 1)" as equivalent to "conditional on A = 1" (as you suggested in a previous comment), then you should regard P(rain | do(grass wet)) as equivalent to P(rain | grass wet). But these are not in fact equivalent, and adding temporal ordering in there doesn't make them equivalent either. P(rain in the past | do(wet grass) in the present) = P(rain in the past), but P(rain in the past | wet grass in the present) != P(rain in the past) .
There is obviously a difference between observational data and experiments.
No, because they're modeling different reality.
Yes! The difference is that experiments involve intervention. I thought the necessity of formalizing the notion of intervention is precisely what was under dispute here.
Well, kinda. I am not sure whether the final output -- the joint densities of outcomes -- will be different in a causal model compared to a properly specified conventional model.
To continue with the same example, it suffers from the expression "wet grass" meaning two different things -- either "I see wet grass" or "I made grass wet". This is your difference between just (a=1) and do(a=1) -- but conventional non-causal modeling doesn't have huge problems with this, it is fully aware of the difference.
And I don't know if it's necessary to formalize intervention. I freely concede that it's useful in certain areas but not so sure that's true for all areas.
I agree with pragmatist's explanation. But let me add a bit more detail to illustrate that a temporal ordering will not save you here. Imagine instead of two variables we have three variables : rain (R), my grass being wet (G1), and my neighbor's grass being wet (G2). Clearly R preceeds both G1, and G2, and G1 and G2 are contemporaneous. In fact, we can even consider G2 to be my neighbor's grass 1 hour in the future (so clearly G1 preceeds G2!).
Also clearly, p(R = yes | G1 = wet) is high, and p(R = yes | G2 = wet) is high, also p(G1 = wet | R = yes) is high, and p(G2 = wet | R = yes) is high.
So by hosing my grass I am making it more likely than my neighbor's grass one hour from now will be wet?
Or, to be more succinct : http://www.smbc-comics.com/index.php?db=comics&id=1994#comic
Yeah, well, I've heard somewhere that correlation does not equal causation :-)
I agree that causal models are useful -- if only because they make explicit certain relationships which are implicit in plain-vanilla regular models and so trip up people on a regular basis.What I'm not convinced of is that you can't re-express that joint density on the outcomes in a conventional way even if it turns out to look a bit awkward.
Here's how this conversation played out.
Lumifer : "can we not express cause effect relationships via conditioning probabilities?"
me : "No: [example]."
Lumifer : "Ah, but this is silly because of time ordering information."
me : "Time ordering doesn't matter: [slight modification of example]."
Lumifer : "Yeah... causal models are useful, but it's not clear they cannot be expressed via conditioning probabilities."
I guess you can lead a horse to water, but you can't make him drink. I have given you everything, all you have to do is update and move on. Or not, it's up to you.
Yes, I'm a picky sort of a horse :-) Thanks for the effort, though.