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The ``true value'' of the mean could be defined
as the mean that results when the coin is flipped
*n* times, when *n* is conceived of as going to infinity.
A more convenient definition of true value
is that the experiment is imagined
as having been done separately
under identical conditions
by an infinite number of people (an ``**ensemble**'').
The ensemble may seem a strange construction;
nonetheless, much literature in statistics and the natural sciences
uses the ensemble idea.
Let us say that the ensemble is defined by
a probability as a function of time.
Then the ensemble idea enables us to define a time-variable mean
(the sum of the values found by the ensemble
weighted by the probabilities)
for, for example, coins that change with time.

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Stanford Exploration Project

10/21/1998