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【文章內(nèi)容簡(jiǎn)介】 ent since evidence for earthquake decreases belief in burglary. and vice versa. 12 Bayes Net Inference ? Given known values for some evidence variables, determine the posterior probability of some query variables. ? Example: Given that John calls, what is the probability that there is a Burglary? Burglary Earthquake Alarm JohnCalls MaryCalls ??? John calls 90% of the time there is an Alarm and the Alarm detects 94% of Burglaries so people generally think it should be fairly high. However, this ignores the prior probability of John calling. 13 Bayes Net Inference ? Example: Given that John calls, what is the probability that there is a Burglary? Burglary Earthquake Alarm JohnCalls MaryCalls ??? John also calls 5% of the time when there is no Alarm. So over 1,000 days we expect 1 Burglary and John will probably call. However, he will also call with a false report 50 times on average. So the call is about 50 times more likely a false report: P(Burglary | JohnCalls) ≈ P(B) .001 A P(J) T .90 F .05 14 Bayes Net Inference ? Example: Given that John calls, what is the probability that there is a Burglary? Burglary Earthquake Alarm JohnCalls MaryCalls ??? Actual probability of Burglary is since the alarm is not perfect (an Earthquake could have set it off or it could have gone off on its own). On the other side, even if there was not an alarm and John called incorrectly, there could have been an undetected Burglary anyway, but this is unlikely. P(B) .001 A P(J) T .90 F .05 15 Types of Inference 16 Sample Inferences ? Diagnostic (evidential, abductive): From effect to cause. – P(Burglary | JohnCalls) = – P(Burglary | JohnCalls ? MaryCalls) = – P(Alarm | JohnCalls ? MaryCalls) = – P(Earthquake | JohnCalls ? MaryCalls) = ? Causal (predictive): From cause to effect – P(JohnCalls | Burglary) = – P(MaryCalls | Burglary) = ? Intercausal (explaining away): Between causes of a mon effect. – P(Burglary | Alarm) = – P(Burglary | Alarm ? Earthquake) = ? Mixed: Two or more of the above bined – (diagnostic and causal) P(Alarm | JohnCalls ? 172。Earthquake) = – (diagnostic and intercausal) P(Burglary | JohnCalls ? 172。Earthquake) = 17 Sample
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