Uncertain Reasoning

Reference: Bratko ed. 3, p. 360-

Sometimes the knowledge in rules is not certain. Rules then may be enhanced by adding information about how certain the conclusions drawn from the rules may be. Our aim in this secion is to describe certainty factors and their manipulation.
Keywords: certainty factor
  • why we might be uncertain
  • measuring uncertainty
  • using certainty factors in rules
  • combining uncertain evidence from more than one source

Certainty Factors

  if   the infection is primary-bacteremia
  and  the site of the culture is one of the sterile sites
  and  the suspected portal of entry is the gastrointestinal tract
  then there is suggestive evidence (0.7) that the infection is bacteroid

Certainty factors have been quantified using various different systems, including linguistics ones (certain, fairly certain, likely, unlikely, highly unlikely, definitely not) and various numeric scales, such as 0-10, 0-1, and -1 to 1. We shall concentrate on the -1 to 1 version.

Certainty factors may apply both to facts and to rules, or rather to the conclusion(s) of rules.

A "Theory" of Certainty

Certainty Factors and Rules


if (P1 and P2) or P3 then C1 (0.7) and C2 (0.3)

Assume CF(P1) = 0.6, CF(P2) = 0.4, CF(P3) = 0.2

CF(P1 and P2) = min(0.6, 0.4) = 0.4

CF(0.4, P3) = max(0.4, 0.2) = 0.4

CF(C1) = 0.7 * 0.4 = 0.28

CF(C2) = 0.3 * 0.4 = 0.12

Combining Multiple CF's