Expert SystemsHandling Uncertainty
Expert systems have to be able to handle uncertainty.
Domain experts may not be 100% certain of their knowledge. ("Global warming is likely to be the cause of rising sea levels").
Information about the problem to be solved may be incomplete or unreliable ("It is a kind of dull, aching pain").
Uncertainty in Output (Advice)
Certainty factors are one simple method of indicating the degree of beief in advice being provided, e.g.
IF clouds are cumulus
THEN advice is it will rain today
CF 40.
If the input is known to be cumulus clouds then the advice it will rain today will be provided, but the user will be instructed that this is only 40% likely to occur.
This allows the develop of the expert system to allow the user to see there is a degree of risk in taking the advice. To this extent, it is a means of the develop protecting themselves from someone trying to blame them for the advice that was generated, acted upon and ultimately proved to be wrong.
Uncertainty in Input
Expert systems should also allow the user to input their degree of uncertainty when asked a particular question.
E,g,
IF surface is free draining
AND retains moisture is true
THEN soil is loamy (0.9).
Assume the user is 60% certain the surface is free draining and 50% certain it retains moisture, then how can these certainty factors for the conditions be applied to the advice from the rule which would be generated with a 90% certainty only if both the conditions are 100% true.
The method applied for your exam is to play cautious. Take the condition of which we are less certain (the 50% for retains moisture rather than 60% for free draining) and multiply this by the certainty factor for the rule to provide the certainty factor for the outcome, i.e. 50% x 90% = 45%. 45% is the minimum degree of confidence that can be generated here. (again, we play cautious. Using the 60% from free draining would produce a certainty factor of 54%. If the advice proves unsound, is it not better to say you were only 45% sure as opposed to 54%)?
In short, the following formula is applied for certainty factors:
CFoutcome = Min(CFcondition1 , CFcondition2, CFconditionn) x CFrule
