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Applied Statistics Using SPSS, STATISTICA, MATLAB and R

Applied Statistics Using SPSS, STATISTICA, MATLAB and R

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51 50 52 − ( k −1)<br />

1 1<br />

p ( k)<br />

= L<br />

= .<br />

52 51 52 − ( k − 2)<br />

52 52<br />

144424443<br />

wrong card in the first k −1<br />

trials<br />

B.1 Discrete Distributions 433<br />

Therefore the r<strong>and</strong>om variable follows a uniform law with n = 52.<br />

B.1.3 Geometric Distribution<br />

Description: Probability of an event occurring for the first time at the kth trial, in a<br />

sequence of independent Bernoulli trials, when it has a probability p of occurrence<br />

in one trial.<br />

Sample space: {1, 2, 3, …}.<br />

Probability function:<br />

g<br />

p<br />

k −1<br />

( k)<br />

≡ P(<br />

X = k)<br />

= ( 1−<br />

p)<br />

p , x∈{1, 2, 3, …} (0, otherwise). B. 4<br />

Distribution function:<br />

G<br />

p<br />

k<br />

∑<br />

i=<br />

1<br />

( k)<br />

= g ( i)<br />

. B. 5<br />

Mean: 1/p.<br />

Variance: (1− p)/p 2 .<br />

0.3<br />

0.25<br />

0.2<br />

0.15<br />

0.1<br />

0.05<br />

0<br />

p<br />

g p (x )<br />

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15<br />

Figure B.3. Geometric probability function for p = 0.25. The mean occurs at x = 4.<br />

Example B. 3<br />

Q: What is the probability that one has to wait at least 6 trials before obtaining a<br />

certain face when tossing a dice?<br />

x

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