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Understanding ChiSquared
• Consider 2 situations:
– Comparing sampled Data directly with the parent
distribution from which it came
– Comparing sampled Data with our best estimate of
that parent distribution
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DOF
Performing a LeastSquares Fit
and Understanding the Result
A complete example
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SUMMARY: Best Fit line with
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Compute ChiSquare for fit
()
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χ
= 1.95 (for example data on previous slide)
DF = Degrees of Freedom
P = Probability of exceeding the tabulated value of
Chisquare (for a given DF)
How do we interpret this value of
ChiSquare?
• We interpret this the following way:
If we were
to repeat the same experiment many times, we
would expect approximately 96% of those
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 Fall '07
 Dobbs
 Approximation, 1 Hour, physicist, 4%

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