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From Algorithms to Z-Scores - matloff - University of California, Davis

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114 CHAPTER 5. CONTINUOUS PROBABILITY MODELS<br />

> 1 − pchisq ( 0 . 3 6 / 0 . 0 4 , 2 )<br />

[ 1 ] 0.01110900<br />

5.5.3.3 Importance in Modeling<br />

This distribution is used widely in statistical applications. As will be seen in our chapters on<br />

statistics, many statistical methods involve a sum <strong>of</strong> squared normal random variables. 7<br />

5.5.4 The Exponential Family <strong>of</strong> Distributions<br />

Please note: We have been talking here <strong>of</strong> parametric families <strong>of</strong> distributions, and in this section<br />

will introduce one <strong>of</strong> the most famous, the family <strong>of</strong> exponential distributions. This should not be<br />

confused, though, with the term exponential family that arises in mathematical statistics, which<br />

includes exponential distributions but is much broader.<br />

5.5.4.1 Density and Properties<br />

The densities in this family have the form<br />

This is a one-parameter family <strong>of</strong> distributions.<br />

fW (t) = λe −λt , 0 < t < ∞ (5.66)<br />

After integration, one finds that E(W ) = 1<br />

1<br />

λ and V ar(W ) = λ2 . You might wonder why it is<br />

cus<strong>to</strong>mary <strong>to</strong> index the family via λ rather than 1/λ (see (5.66)), since the latter is the mean. But<br />

this is actually quite natural, for the reason cited in the following subsection.<br />

5.5.4.2 R Functions<br />

Relevant functions for a uniformly distributed random variable X with parameter λ are<br />

• pexp(q,lambda), <strong>to</strong> find P (X ≤ q)<br />

• qexp(q,lambda), <strong>to</strong> find c such that P (X ≤ c) = q<br />

• rexp(n,lambda), <strong>to</strong> generate n independent values <strong>of</strong> X<br />

7 The motivation for the term degrees <strong>of</strong> freedom will be explained in those chapters <strong>to</strong>o.

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