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Errors, Failures, and Risks. Items to Discuss. Failures and Errors in Computer Systems Case Study: The Therac-25 Increasing Reliability and Safety Dependence, Risk, and Progress. Failures and Errors in Computer Systems.

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Errors failures and risks l.jpg

Errors, Failures, and Risks


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Items to Discuss

  • Failures and Errors in Computer Systems

  • Case Study: The Therac-25

  • Increasing Reliability and Safety

  • Dependence, Risk, and Progress


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Failures and Errors in Computer Systems

  • Most computer applications are so complex it is virtually impossible to produce programs with no errors

  • The cause of failure is often more than one factor

  • Computer professionals must study failures to learn how to avoid them

  • Computer professionals must study failures to understand the impacts of poor work


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Failures and Errors in Computer SystemsIndividual Problems

  • Billing errors

    • A 101-year man suddenly had a auto insurance rate that was tripled.

    • Chicago cat owners were billed for failure to register Dachshunds.

  • Inaccurate and misinterpreted data in databases


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Failures and Errors in Computer SystemsIndividual Problems

  • Inaccurate and misinterpreted data in databases

    • 2000 election, Florida had lists that would not allow people to vote because their names were similar to convicted felons.

    • People could be misrepresented by sex offender databases.

    • TSA have indicated that more than 30,000 people have been mistakenly matched to terrorist watch lists at airports and border crossings.


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Failures and Errors in Computer SystemsIndividual Problems

  • Inaccurate and misinterpreted data in databases

    • Large population where people may share names

    • Automated processing may not be able to recognize special cases

    • Overconfidence in the accuracy of data

    • Errors in data entry

    • Lack of accountability for errors


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Failures and Errors in Computer Systems System Failures

  • AT&T

    • Lost phone service for voice and data for nine-hours because of error in a 3-line change to a 4-million line program.

  • Tokyo Stock Exchange, NASDAQ, London Stock Ex.

    • A software upgrade in Tokyo shutdown the trading floor.

    • An update to Charles Schwab Corp. virtually shutdown NASDAQ for 2-hours.

    • London had a software glitch freeze that system for 8-hours the last day of the tax year.

  • Businesses have gone bankrupt after spending huge amounts on computer systems that failed.


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Failures and Errors in Computer Systems System Failures

  • Voting system in 2000 presidential election because of outdated voting machines.

  • Denver Airport

    • This airport is extremely large and the underground baggage system consists of 22-miles of underground track, which refused to function correctly.


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Failures and Errors in Computer Systems System Failures

  • Mars Climate Orbiter misinterpreted altitude data, causing the orbiter crash.


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Failures and Errors in Computer Systems System Failures

  • Ariane 5 Rocket incorrectly sent data to the rocket control system after a hardware upgrade, causing it to immediate loose control after launch.


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Failures and Errors in Computer SystemsReasons

Denver Airport:

  • Baggage system failed due to real world problems, problems in other systems and software errors

  • Main causes:

    • Time allowed for development was insufficient

    • Denver made significant changes in specifications after the project began


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Failures and Errors in Computer SystemsReasons

High-level Causes of Computer-System Failures:

  • Lack of clear, well thought out goals and specifications

  • Poor management and poor communication among customers, designers, programmers, etc.

  • Pressures that encourage unrealistically low bids, low budget requests, and underestimates of time requirements

  • Use of very new technology, with unknown reliability and problems

  • Refusal to recognize or admit a project is in trouble


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Failures and Errors in Computer SystemsReasons

Safety-Critical Applications:

  • A-320: "fly-by-the-wire" airplanes (many systems are controlled by computers and not directly by the pilots)

    • Between 1988-1992 four planes crashed

  • Air traffic control is extremely complex, and includes computers on the ground at airports, devices in thousands of airplanes, radar, databases, communications, and so on - all of which must work in real time, tracking airplanes that move very fast

  • In spite of problems, computers and other technologies have made air travel safer


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Case Study: The Therac-25

Therac-25 Radiation Overdoses:

  • Massive overdoses of radiation were given; the machine said no dose had been administered at all

  • Caused severe and painful injuries and the death of three patients

  • Important to study to avoid repeating errors

  • Manufacturer, computer programmer, and hospitals/clinics all have some responsibility


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Case Study: The Therac-25

Software and Design problems:

  • Re-used software from older systems, unaware of bugs in previous software

  • Weaknesses in design of operator interface

  • Inadequate test plan

  • Bugs in software

    • Allowed beam to deploy when table not in proper position

    • Ignored changes and corrections operators made at console


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Case Study: The Therac-25

Why So Many Incidents?

  • Hospitals had never seen such massive overdoses before, were unsure of the cause

  • Manufacturer said the machine could not have caused the overdoses and no other incidents had been reported (which was untrue)

  • The manufacturer made changes to the turntable and claimed they had improved safety after the second accident. The changes did not correct any of the causes identified later


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Case Study: The Therac-25 (cont.)

Why So Many Incidents? (cont.)

  • Recommendations were made for further changes to enhance safety; the manufacturer did not implement them

  • The FDA declared the machine defective after the fifth accident

  • The sixth accident occurred while the FDA was negotiating with the manufacturer on what changes were needed


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Increasing Reliability and Safety

What goes Wrong?

  • Design and development problems

  • Management and use problems

  • Misrepresentation, hiding problems and inadequate response to reported problems

  • Insufficient market or legal incentives to do a better job

  • Re-use of software without sufficiently understanding the code and testing it

  • Failure to update or maintain a database


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Increasing Reliability and SafetyProfessional techniques

  • Importance of good software engineering and professional responsibility

  • User interfaces and human factors

    • Feedback

    • Should behave as an experienced user expects

    • Workload that is too low can lead to mistakes

  • Redundancy and self-checking

  • Testing

    • Include real world testing with real users


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Increasing Reliability and SafetyLaw, Regulation and Markets

  • Criminal and civil penalties

    • Provide incentives to produce good systems, but shouldn't inhibit innovation

  • Warranties for consumer software

    • Most are sold ‘as-is’

  • Regulation for safety-critical applications

  • Professional licensing

    • Arguments for and against

  • Taking responsibility


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Dependence, Risk, and ProgressRisk and Progress

  • Many new technologies were not very safe when they were first developed

  • We develop and improve new technologies in response to accidents and disasters

  • We should compare the risks of using computers with the risks of other methods and the benefits to be gained


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Looking at things in new ways.


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Information, Knowledge, and JudgmentEvaluating Information on the Web

  • Expert information or ‘wisdom of the crowd’?

    • Daunting amount of information on the web, much of this information is not correct

    • Search engines are replacing librarians, but Web sites are ranked by popularity, not by expert evaluation

    • Wisdom of the crowd - ratings by public of website

    • If millions participate, the results will be useful


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Information, Knowledge, and JudgmentEvaluating Information on the Web

  • Wikipedia:

    • Written by volunteers, some posts are biased and not accurate

    • Although anyone can write, most people do not

    • Those that do typically are educated and experts


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Information, Knowledge, and JudgmentEvaluating Information on the Web

  • Wisdom of the crowd

    • Problems of unreliable information are not new

    • The Web magnifies the problems

    • Rating systems are easy to manipulate

  • Vulnerable viewers

    • Less educated individuals

    • Children

  • Responsibilities of site operators

    • Should identify user-supplied content

    • Make clear which information has been verified


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Information, Knowledge, and JudgmentWriting, Thinking and Deciding

  • New tools have displaced skills that were once important

  • Abdicating responsibility

    • People willing to let computers do their thinking

    • Reliance on computer systems over human judgment may become institutionalized

    • Fear of having to defend your own judgment if something goes wrong


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Information, Knowledge, and Judgment Computer Models

  • Evaluating Models

    • How well do the modelers understand the underlying science or theory?

    • Models necessarily involve assumptions and simplifications of reality

    • How closely do the results or predictions correspond with the results from physical experiments or real experience?


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Information, Knowledge, and Judgment Computer Models

  • Why models may not be accurate

    • We might not have complete knowledge of the system we are modeling

    • The data describing current conditions or characteristics may be incomplete of inaccurate

    • Computing power may be inadequate for the complexity of the model

    • It is difficult, if not impossible, to numerically quantify variables that represent human values and choices


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The "Digital Divide”Trends in Computer Access

  • New technologies only available to the wealthy

  • The time it takes for new technology to make its way into common use is decreasing

  • Cost is not the only factor; ease of use plays a role

  • Entrepreneurs provide low cost options for people who cannot otherwise afford something

  • Government funds technology in schools

  • As technology becomes more prevalent, the issues shift from the haves and have-nots to level of service


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The "Digital Divide”The Global Divide and the Next Billion Users

  • Approximately one billion people worldwide have access to the Web; approximately five billion do not

  • Non-profit organizations and huge computer companies are spreading computer access to people in developing countries

  • Bringing new technology to poor countries is not just a matter of money to buy equipment; PCs and laptops must work in extreme environments

  • Some people actively working to shrink the digital divide emphasize the need to provide access in ways appropriate to the local culture


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Evaluations of the Impact of Computer Technology

The Neo-Luddite View of Computers

  • Computers cause massive unemployment

  • No real need (We use technologies because they are there, not because they satisfy real needs)

  • Computers cause social inequity

  • Benefit big business and the government

  • Do little or nothing to solve real problems

  • Computers separate humans from nature and destroy the environment


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Evaluations of the Impact of Computer Technology (cont.)

Accomplishments of Technology

  • Prices of food are down and raw materials are abundant

  • Real buying power is up

  • Food supplies and GDP are growing faster than the population

  • Dramatic impact on life expectancy

  • Assistive technologies benefit those with disabilities


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Making Decisions About Technology

The Difficulty of Prediction

  • Each new technology finds new and unexpected uses

  • The history of technology is full of wildly wrong predictions


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