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Final Answers
© 2000-2020 Gérard P. Michon, Ph.D.

Bayesian Statistics
On the Probability of Causes

When the facts change, I change my opinion.
What do you do, sir?

John Maynard Keynes (1883-1946)
Michon
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Related articles on this site:

Related Links (Outside this Site)

Theory of Probability (1939) by Sir Harold Jeffreys (1891-1989)

The Mathematics of Changing your Mind by John Allen Paulos (2011)
Bayes' theorem by Kalid ("Better Explained").
The Mascots of Bayesian Statistics by Rasmus Bååth (2013年12月26日).

Why Judea Pearl is only half-Bayesian by Andrew Gelman.
Causality by Judea Pearl (Second Edition, 2009) [2000 review]

Wikipedia : Bayesian Statistics | Quantum Bayesianism

Videos : How one equation changed the way I think by Julia Galef.
A visual guide to Bayesian thinking by Julia Galef (2015年07月16日).
Simpson's Paradox (14:05) by David Louapre (in French, 2015年04月03日).
The Bayesian Trap (10:36) by Derek Muller (Veritasium, 2017年04月05日).
Asymmetric Intelligibility in Natural Languages (8:31) by Josh (2018年06月29日).
Bayesian Statistics (13:47) Jannah Fry&anbsp; & Matt Parker (2019年03月29日).

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Bayesian Statistics & Probabilistic Inferences


(2013年10月30日) Bayesian Symmetry: P(A,B) = P(B,A). Is it obvious?
The relationship between joint probability and conditional probability.

In practice, the Bayesian formula (which we shall presently introduce) is often applied to an uncertain hypothesis A and a "test" B for it, relating the a priori probability P(A) to the a posteriori probability P(A|B).

However, both A and B can simply be considered to be events placed on the same footing, which do not play different mathematical rôles:

  • P(A) denotes the probability of A. P(B) is the probability of B.
  • P(A,B) is the joint probability that A and B both occur.
  • P(A|B) is the conditional probability of A when B does occur.

P(A|B) is read as the probability of "A knowing B". The following holds:

P(A|B) P(B) = P(A,B) = P(B,A) = P(B|A) P(A)

The fact that togetherness is symmetric [i.e., P(A,B) = P(B,A) ] is crucial in the above, which may serve as a proof of the following Bayes' formula of inference, upon which Bayesian statistics is based.

The formula itself is arguably due to Laplace (1812) as Thomas Bayes (1702-1761) didn't bother with such a formal expression. Neither did Richard Price who edited the work of Bayes posthumously (1763).
Bayes' Theorem (Bayes' Formula)
P(A|B) = P(B|A) P(A)
vinculum
P(B)

This is consistent with a classical description of reality where probabilities are expressed in terms of classical events which could be, among other possibilities, independent or mutually exclusive.

There are probabilistic systems which cannot be described in classical terms (no two events are ever independent or mutually exclusive). The quantum universe (which we live in) is one such system, where the above foundational relation of Bayesian statistics is neither obvious nor true, as experimental violations of Bell's inequality demonstrate.

Bayes' theorem is just true in a self-consistent system where probability is a classical measure satisfying the following axiom of measure theory for the subsets of some universal set E of probability 1.

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Thomas Bayes (c.1701-1761) | Richard Price (1723-1791) | Laplace (1749-1827)

Wikipedia : Bayes' theorem | Bayesian probability | Bayesian inference


(2015年01月01日) The Bayesian Universe.
Probabilities quantify beliefs.

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Wikipedia : Sequential analysis | Decision theory


(2017年04月18日) Yule-Simpson paradox. Confusion factors.
Ignoring true causes can make correlations meaningless.

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Udny Yule (1871-1951) | Edward H. Simpson (1922-)

Machine Learning Methods (10:40) by Uwe Aickelin (Computerphile, 2015年09月02日).
Anti-learning (7:50) by Uwe Aickelin (Computerphile, 2015年09月23日).

Wikipedia : Simpson's Paradox

Video : Simpson's Paradox (4:39) by Henry Reich (Minute Physics, 2017年10月24日).


(2020年03月16日) Raw product ratings
How do they compare to each other?

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A Bayesian view of Amazon Resellers by John D. Cook (2011年09月27日).

Which rating is mathematically better? (12:33) by Grant Sanderson (2020年03月15日).


(2013年11月01日) Quantum Theory is not Bayesian.
Bayes' formula doesn't apply to the probabilities of quantum observations.

In quantum theory, events that can be placed on the same footing correspond to commuting observables. Most pairs of observables do not commute and joint probabilities are strangely defined.

Wikipedia : Quantum Bayesianism | Quantum information


(2013年10月30日) Is the human brain a Bayesian engine?

No, it's not. At least not a perfect one. If it was, then it wouldn't be capable of irrational decisions.

Arguably, the biology of the brain involves processes that entail voting and the associated irrational nontransitivy stemming from Condorcet's paradox.

This is not to say, thankfully, that humans cannot consciously revise their beliefs or opinions when presented with new evidence. It merely goes to say that it takes some effort to do so. Just as it takes some effort to practice mathematics and obtain flawless results, in spite of our natural tendencies for intuition, preconceptions and mistakes.


(2015年03月30日) Causality
Post Hoc, Ergo Propter Hoc

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