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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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where D <strong>and</strong> S st<strong>and</strong> for disease <strong>and</strong> symptoms respectively. Normally, P(S)<br />

is unknown. So to overcome the difficulty the following formalism is used in<br />

practice.<br />

We compute: P(¬ D / S) = P(S / ¬ D). P(¬ D) / P(S). (9.3)<br />

Now from expressions (9.2) <strong>and</strong> (9.3) we find,<br />

P (D / S ) P (S / D) P ( D )<br />

------------ = ------------ x -------------<br />

P (¬ D /S) P (S /¬ D) P (¬ D)<br />

i.e., O (D / S) = L (S / D) x O (D)<br />

where O <strong>and</strong> L denotes the odds of an event <strong>and</strong> the likelihood ratio [1].<br />

However, in some circumstances, where P (S) is known, we can directly<br />

use the Bayes’ theorem.<br />

In our formal notations, consider the set H <strong>and</strong> E to be partitioned into<br />

subsets, vide fig. 9.1.<br />

H n Em<br />

H 1 …….. E 1 E 2 ……<br />

H 2<br />

(a) (b)<br />

Fig. 9.1: (a) Partitioned Hj , 1≤ j ≤ n <strong>and</strong> (b) Partitioned Ei , 1≤ i ≤ m.<br />

The expression (9.1) can now be extended to take into account the joint<br />

occurrence of Ei with all Hj for 1 ≤ j ≤ n.<br />

P (Ei )<br />

= P (Ei ∩ H1) + P (Ei ∩ H2) + ... + P (Ei ∩ Hn)<br />

= P (Ei / H1) x P (H1) + P (Ei / H2) x P (H2) +… + P (Ei / Hn ) x P (Hn)<br />

(9.5)

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