"""This file contains code for use with "Think Bayes",
by Allen B. Downey, available from greenteapress.com
Copyright 2012 Allen B. Downey
License: GNU GPLv3 http://www.gnu.org/licenses/gpl.html
"""
"""This file contains a partial solution to a problem from
MacKay, "Information Theory, Inference, and Learning Algorithms."
Exercise 3.15 (page 50): A statistical statement appeared in
"The Guardian" on Friday January 4, 2002:
When spun on edge 250 times, a Belgian one-euro coin came
up heads 140 times and tails 110. 'It looks very suspicious
to me,' said Barry Blight, a statistics lecturer at the London
School of Economics. 'If the coin were unbiased, the chance of
getting a result as extreme as that would be less than 7%.'
MacKay asks, "But do these data give evidence that the coin is biased
rather than fair?"
"""
import thinkbayes
import thinkplot
class Euro(thinkbayes.Suite):
"""Represents hypotheses about the probability of heads."""
def Likelihood(self, data, hypo):
"""Computes the likelihood of the data under the hypothesis.
hypo: integer value of x, the probability of heads (0-100)
data: string 'H' or 'T'
"""
x = hypo / 100.0
if data == 'H':
return x
else:
return 1-x
class Euro2(thinkbayes.Suite):
"""Represents hypotheses about the probability of heads."""
def Likelihood(self, data, hypo):
"""Computes the likelihood of the data under the hypothesis.
hypo: integer value of x, the probability of heads (0-100)
data: tuple of (number of heads, number of tails)
"""
x = hypo / 100.0
heads, tails = data
like = x**heads * (1-x)**tails
return like
def Version1():
suite = Euro(xrange(0, 101))
heads, tails = 140, 110
dataset = 'H' * heads + 'T' * tails
for data in dataset:
suite.Update(data)
return suite
def Version2():
suite = Euro(xrange(0, 101))
heads, tails = 140, 110
dataset = 'H' * heads + 'T' * tails
suite.UpdateSet(dataset)
return suite
def Version3():
suite = Euro2(xrange(0, 101))
heads, tails = 140, 110
suite.Update((heads, tails))
return suite
def main():
suite = Version3()
print suite.Mean()
thinkplot.Pmf(suite)
thinkplot.Show()
if __name__ == '__main__':
main()
