CptS 401, Fall 2010 11/2/10

Chapter 8: Errors, Failures, and Risk

See Dilbert cartoons about errors, failures, and risk:

http://dilbert.com/strips/comic/2010-05-20/

http://dilbert.com/strips/comic/2006-12-10/\

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CptS 401, Fall 2010 11/2/10

Reminders

• Video scene due tonight at 11:59– Post to YouTube as “unlisted”– Submit .txt file to OSBLE with YouTube link

• Today’s reading quiz covers 8.1 and 8.2

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CptS 401, Fall 2010 11/2/10

Quiz Question 1

The cause of computer system failures can be attributed toA. faulty designB. sloppy implementationC. poor user interfacesD. All of the aboveE. None of the above

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CptS 401, Fall 2010 11/2/10

Answer : E(See p. 405)

• Computer software is inherently complex and will contain bugs– How much risk should we accept? – Are some risks just tradeoffs for the benefits that the software

provides? • Three useful perspectives

– Computer users, who need to understand limitations of technology

– Computer professionals, who need become better at what they do

– Educated members of society, who make personal, social, legal and political decisions based on risks of computers

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CptS 401, Fall 2010 11/2/10

Quiz Question 2

The following is not a factor that contributes to the problems that computer databases cause for people:A. A large populationB. Automated processing without human

interventionC. Inaccurate biometric dataD. Errors in data entryE. Lack of accountability for errors

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CptS 401, Fall 2010 11/2/10

Answer: CSeveral Factors Contribute to Database Problems

• A large population (many with similar names)• Automated processing can’t always recognize

special cases or doesn’t use common sense• Overconfidence in accuracy of data• Human errors in data entry• Data are not updated or maintained• Humans aren’t accountable for problems

(blame it on computer!)

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CptS 401, Fall 2010 11/2/10

Quiz Question 3

In the context of Ch. 8, RSI stands forA. Redundant System InputsB. Recalibrated System IntegrityC. Repetitive Safety IsometricsD. Redundant Safety InformationE. None of the above

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CptS 401, Fall 2010 11/2/10

Answer: RSI = Repetitive Strain Injuries (E)(see gray box on p. 411)

• Have you experienced pain from using a keyboard or mobile device for extended periods of time?A: YesB: No

• What can you do to prevent this?

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CptS 401, Fall 2010 11/2/10

Quiz Question 4

A strategy mentioned by the book for making electronic voting machines less vulnerable to fraud and sabotage isA. requiring the machines to be housed in a

locked cabinetB. requiring the software they use to be publicC. keeping the software they use secretD. All of the aboveE. None of the above

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CptS 401, Fall 2010 11/2/10

Answer: B (see p. 416)Electronic Voting has Pros and Cons

• With your group discuss the pros and cons of electronic voting for about 90 seconds

• Can you cite recent elections in which it worked or didn’t work?

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CptS 401, Fall 2010 11/2/10

Quiz Question 5

The book discusses the failures of complex computer systems at three different airports. What was a common reason all three failed?A. They had not undergone sufficient testingB. They were unable to handle incorrect user

inputC. They were too complex to work properlyD. None of the above

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CptS 401, Fall 2010 11/2/10

Answer: ASee pp. 418-419

• What were the three airports?• There was immense political pressure to deploy

these systems quickly• Not enough time was allocated for thorough testing

of the software, nor was adequate time allocated to properly train human workers in the use of the software

• Cite examples of abandoned computer systems in which millions or even billions of dollars were invested.

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CptS 401, Fall 2010 11/2/10

Quiz Question 6

systems are , thus requiring special adaptations to work with

.A. Operating, difficult to program, antivirus

softwareB. legacy, out-of-date, more modern systemsC. safety-critical, highly complex, computer

softwareD. Voting, proprietary, elections

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CptS 401, Fall 2010 11/2/10

Answer: B (see p. 420)Legacy Systems Have Caused Many Problems

• Large computer systems were originally used primarily in the banking and airline industries, and also by the government

• It is difficult for these systems to go “off-line” so that they can be replaced by new systems

• Hence, they often continue to be used, even though they are outdated, difficult to use, and difficult to maintain

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CptS 401, Fall 2010 11/2/10

Quiz Question 7

In the first decade of the 21st century, there was one fatal accident per commercial flights. This was than the rate of the previous decade. A. 10,000; higherB. 100,000; lowerC. 1 million, higherD. 4 million, lowerE. None of the above

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CptS 401, Fall 2010 11/2/10

Answer: DAirline Travel is Astonishingly Safe!

• Several computer-based innovations have contributed to airline travel’s improving safety record (60% safer than a decade ago)– Name two mentioned in the book

• But: in times of crisis, should a pilot trust these systems or her own instincts?

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CptS 401, Fall 2010 11/2/10

Quiz Question 8

The following is true of the Therac-25 case:A. It’s a shining example of a device that was built

with safety in mind.B. It teaches dramatic lessons about the negative

impacts of poorly-designed software on human life.

C. It shows that the known problems of a device can be avoided by competent operators of the device.

D. None of the above

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CptS 401, Fall 2010 11/2/10

Question 8 Answer

B. (See p. 425)

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CptS 401, Fall 2010 11/2/10

Quiz Question 9

Why did the Therac-6 and Therac-20, both predecessors to the Therac-25, not have the same safety problems as the Therac-25?A. They were generally used by operators with more

skill.B. They didn’t have as strong an electron beam.C. They were controlled by better-designed software.D. They had a hardware interlock mechanism that

prevented the beam from firing in unsafe conditions.E. Non of the above

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CptS 401, Fall 2010 11/2/10

Quiz Question 9 Answer

D. See p. 426

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CptS 401, Fall 2010 11/2/10

Quiz Question 10

One of the software errors that caused the Therac-25 malfunctions was due toA. A variable being incremented instead of set

to a fixed value.B. A loop iterating one too many timesC. An erroneous conditional checkD. All of the aboveE. None of the above

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CptS 401, Fall 2010 11/2/10

Quiz Question 10 Answer

A. (See p. 427)What was the other major software error?

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CptS 401, Fall 2010 11/2/10

Discussion Exercise Assignments

• 8.36:– Hospital that used machine

• Prosecuting attorney: Error 404, Discotech, Tier III, Pac4, • Defense attorney: Breakfast Club, CAAK, Skynet, Prestige

Worldwide

– Company that made the machine• Prosecuting attorney: FAM, Pangea, CADD, Thizzle• Defense attorney: Brandon, Rock-It, Sponge Bob, SPAMD

– Programmer who wrote the software• Prosecuting attorney: Batman Symbol, Skillz that Killz, Hide ‘Yo Kids• Defense attorney: Emoticons, Error 403 Forbidden, SkyBlue, iTeam

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