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Applied Statistics Using SPSS, STATISTICA, MATLAB and R

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446 Appendix B - Distributions<br />

1.2<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

γ a ,p (x )<br />

a =1, p =1<br />

a =1.5, p =2<br />

a =1, p =2<br />

0<br />

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8x<br />

Figure B.12. Gamma density functions for three different pairs of a, p. Notice that<br />

p works as a shape parameter <strong>and</strong> a as a scale parameter.<br />

Example B. 12<br />

Q: The lifetime in years of a certain model of cars, before a major motor repair is<br />

needed, follows a gamma distribution with a = 0.2, p = 3.5. In the first 6 years,<br />

what is the probability that a given car needs a major motor repair?<br />

A: Γ0.2,3.5(6) = 0.066.<br />

B.2.6 Beta Distribution<br />

Description: The Beta distribution is a continuous generalization of the binomial<br />

distribution.<br />

Sample space: [0, 1].<br />

Density function:<br />

1 p−1<br />

q−1<br />

β p,<br />

q ( x)<br />

= x ( 1−<br />

x)<br />

, x∈[0, 1] (0, otherwise), B. 24<br />

B(<br />

p,<br />

q)<br />

Γ(<br />

p)<br />

Γ(<br />

q)<br />

with B ( p,<br />

q)<br />

= , p,<br />

q > 0 , the so-called beta function.<br />

Γ(<br />

p + q)<br />

Distribution function:<br />

x<br />

p, q ( ∫ β<br />

−∞<br />

p,<br />

q<br />

B. 25<br />

Β x)<br />

= ( t)<br />

dt<br />

Mean: µ = p /( p + q)<br />

. The sum c = p + q is called concentration parameter.<br />

2<br />

2<br />

Variance: = pq / [ ( p + q)<br />

( p + q + 1)<br />

] = µ ( 1−<br />

µ ) /( c + 1)<br />

σ .

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