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positions came up “1” if incorrect, otherwise
zero. However, it was stored in ONE BYTE.
Every 256th time the routine was called, the
flag overflowed and showed a value of zero
anyhow. (Subtle, intermittent bug; hard to
❚ Other bugs sometimes allowed the machine to
ignore changes or corrections made by the
operator (edits not passed in real time).
CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 20 10 Spring 2000 - Lecture 7 Therac 25: What went wrong?
❚ Manufacturer was informed of the
incidents (several times) but responded
that the machine could not possibly be
responsible for the massive overdoses.
❚ Later, some machines removed from
service and modified by manufacturer;
but did not catch ALL the bugs before
returned to service.
CIS 350 Lecture 7 21 Therac 25: What went wrong?
❚ This case illustrates many of the things
that a responsible, ethical company would
not do. CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 22 11 Spring 2000 - Lecture 7 Expert Systems In Medicine
❚ Expert systems are an applied type of
❚ To develop an expert system, AI
researchers spend years picking the
brains of human experts, extracting their
knowledge and trying to understand the
way it is organized CIS 350 Lecture 7 23 An Expert System Includes
1. A problem statement with a goal: limited
domain-2. A knowledge base
3. A problem solving strategy that
determines which knowledge to apply and
constructs the lines of reasoning.
("heuristics on next slide)
CIS 350 Lecture 7 © Roxanne Hiltz and Eli Rohn 24 12 Spring 2000 - Lecture 7 An Expert System Includes
Heuristics are intellectual rules of
thumb, or the method by which human
experts make "good guesses."
4. A friendly interface that is "natural" to
the user CIS 350 Lecture 7 25 Examples Of Expert Systems:
❚ MYCIN is the classic example of an
❙ It was created around 1972 to diagnose and propose
treatments for bacterial infections.
❙ It uses a probabilistic form of backward rule chaining,
where assertions are tagged with certainty factors of
–1 (false), through 0 (balanced) to 1 (true) ❚ PUFF helps diagnose lung pr...
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This document was uploaded on 03/31/2014 for the course CS 350 at NJIT.
- Spring '14