From: Amara Graps (amara@amara.com)
Date: Wed Jun 18 2003 - 01:02:34 MDT
(ccing extropians too, only because this topic has appeared there before)
"bzr" <bzr@csd.net>, Sun, 15 Jun 2003:
>However, perhaps the easiest way to see that the Bayesian framework won't do
>as a comprehensive framework for science, and why it assuredly can't proxy
>as the whole of a philosophy of science, is to consider this problem:
>We have a penny. We toss it. What are the odds that we'll get heads?
>The answer: 0
>Zero? Yes. This is very counterintuitive, admittedly, particularly for
>statisticians. However, the truth is there are no "odds" here at all.
>Penny tossing is deterministic.
>That being the case, if we are appraised of all the initial
>conditions of the toss, and possess a complete knowledge of the laws
>of physics, then we can predict with certainty what we will get
>(heads,or tails, or, very rarely, a coin on edge). Even more
>interestingly (and counterintuitively), without any knowledge of
>statistics OR knowlege of physics we can be sure that, provided the
>test surface is flat, we will get heads, tails, or a coin on its
>edge.
I think that using determinism in this way is putting up a smoke
screen in addition to missing the large picture of how scientists
intuitively do science. You have a real experiment, so it is
physical, and all propositions are testable. How do you define
determinism for this system? Your determinism is based on a model of
some physics, is it not? No matter how 'deterministic' something may
be, your prediction for the outcome of the coin toss is based on
data and a model and what other information you have about that
system. A Bayes discussion is always in the realm of epistemology,
i.e. how we know what we know.
Humans never know how nature _is_. All humans can do is make an
abstract physical description of nature. Scientific studies are how
we are able to process information in order to say some things about
that nature. Bayesian concepts makes this process explicit. A
Bayesian perspective of science says that any theory about reality
can have no consequences testable by us, unless that theory can also
describe what humans can see and know. Models, data, prior
information, in other words.
Note also how causality takes a side seat. A logical relationship
between the event (and their probabilities) does not imply a causal
(physical) relationship between the events. Sometimes Bayesians call
this the Mind Projection Fallacy, which is behind a huge number of
misconceptions and 'paradoxes' in mathematics (set theory,
information theory, Fourier transform,...) physics (quantum and
relativistic physics, potential, ...) philosophy (Bohr, Einstein,
Bohm, Popper, Penrose, ...).
Bayes Theorem is only a multiplication rule of probability theory,
which shows a relationship between a posterior probability, a
likelihood of data to model, and prior probability. The prior
probability and posterior probability are not necessarily related in
time. These concepts show just a different relationship to the data
to be analyzed. The Bayesian methodologies approach the scientific
inference from "first principles", grasping an n-parametric event
directly with an n-dimensional posterior probability distribution.
>The question of why statistical analysis "works" (to the extent that
>it does, and given an initial state of ignorance), or indeed the
>question of what conditions must pertain in order for statistical
>analysis to be appropriate, is not itself answerable by further
>statistical analysis.
No.
Some history. The Bayesian probabilistic ideas have been around
since the 1700s. Bernoulli, in 1713, recognized the distinction
between two definitions of probability: (1) probability as a measure
of the plausibility of an event with incomplete knowledge, and (2)
probability as the long-run frequency of occurrence of an event in a
sequence of repeated (sometimes hypothetical) experiments. The
former (1) is a general definition of probability adopted by the
Bayesians. The latter (2) is called the "frequentist" view,
sometimes called the "classical", "orthodox" or "sampling theory"
view.
Scientists who rely on frequentist definitions, while assigning
their uncertainties for their measurements, should be careful. The
concept of sampling theory, or the statistical ensemble, in
astronomy, for example, is often not relevant. A gamma-ray burst is
a unique event, observed once, and the astronomer needs to know what
uncertainty to place on the one data set he/she actually has, not on
thousands of other hypothetical gamma-ray burst events. And
similarly, the astronomer who needs to assign uncertainty to the
large-scale structure of the Universe needs to assign uncertainties
based on _our_ particular Universe, because there are not similar
Observations in each of the "thousands of universes like our own."
The version of Bayes' Theorem that statisticians use today is
actually the generalized version due to Laplace. One particularly
nice example of Laplace's Bayesian work was his estimation of the
mass of Saturn, given orbital data from various astronomical
observatories about the mutual perturbations of Jupiter and Saturn,
and using a physical argument that Saturn's mass cannot be so small
that it would lose its rings or so large that it would disrupt the
Solar System. Laplace said, in his conclusion, that the mass of
Saturn was (1/3512) of the solar mass, and he gave a probability of
11,000 to 1 that the mass of Saturn lies within 1/100 of that value.
He should have placed a bet, because over the next 150 years, the
accumulation of data changed his estimate for the mass of Saturn by
only 0.63% ...
More references that might be useful:
General for scientists: (article)
A.L. Graps, "Probability Offers Link Between Theory and Reality,"
Scientific Computing World, October 1998.
Focusing more on epistemology: (book)
_Scientific Reasoning: The Bayesian Approach_ by Colin Howson and Peter
Urbach, 1989, Open Court Publishing.
Focusing on implementation: (books)
_Bayesian Statistics_ (2nd edition) by Peter M. Lee, Oxford
University Press, 1997.
_Data Analysis: A Bayesian Tutorial_, Sivia, D.S., Clarendon Press:
Oxford, 1996.
Martz, Harry and Waller, Ray, chapter: "Bayesian Methods" in
_Statistical Methods for Physical Science_, Editors: John L.
Stanford and Stephen Vardeman [Volume 28 of the Methods of
Experimental Physics], Academic Press, 1994, pg. 403-432.
Other useful papers on the web:
Epistemology Probabilized by Richard Jeffrey
http://www.princeton.edu/~bayesway/
Edwin Jaynes: Probability
http://bayes.wustl.edu/
"Probability in Quantum Theory",
"Clearing up Mysteries- the Original Goal".
"Role and Meaning of Subjective Probability: Some Comments
on Common Misconceptions." by Giulio D'Agostini
http://zeual1.roma1.infn.it/~agostini/prob+stat.html
Amara
-- ******************************************************************** Amara Graps, PhD email: amara@amara.com Computational Physics vita: ftp://ftp.amara.com/pub/resume.txt Multiplex Answers URL: http://www.amara.com/ ******************************************************************** "The understanding of atomic physics is child's play compared with the understanding of child's play." -- David Kresch
This archive was generated by hypermail 2.1.5 : Wed Jun 18 2003 - 01:13:07 MDT