## Processing Certainty Factors

Two basic issues:
• When to decide confidence is too low for rule to apply
• How to compute certainty factors in rules

#### Certainty threshold

Within an inference engine, a threshold value typically is chosen for certainty factors
• Values below threshold indicate conclusion is too uncertain to hold, so rule not used
• Values above threshold indicate conclusion has acceptable certainty

#### Computing certainty factors

• Rules state that a conclusion applies with given confidence, given hypotheses.
• What confidence should be concluded if hypotheses are not completely certain?
• Also, what confidence should be concluded if rule has multiple hypotheses, each of which is not fully certain?
Several approaches are possible to combine certainty factors.
Here is one approach, patterned after TMycin by Gordon Novak

Proceed in 2 steps

1. Determine confidence in rule conditions being met
2. Base confidence on rule conclusions on condition confidence and rule confidence factor

#### Confidence in Rule Conditions

• If only one factor in condition, that gives the confidence
• If several factors, connected with OR, take maximum confidence
• If several factors, connected with AND, take minimum confidence

#### Confidence in conclusions, given hypotheses

• Multiply rule confidence by hypothesis confidence
Notes:
• Computed certainty factors can get low quickly, so one must be somewhat careful in choosing thresholds
• A threshold confidence of 0.2 or so may be appropriate!

 created May 13, 2003 last revised May 13, 2003 previous   next