Chapter 7: Computer Reliability

Copyright © Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 7: Computer Reliability

Ethics for the Information AgeForth Edition

by Michael J. Quinn

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Chapter Overview

• Introduction• Data-entry or data-retrieval errors• Software and billing errors• Notable software system failures• Therac-25• Computer simulations• Software engineering• Software warranties

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Introduction

• Computer systems are sometimes unreliable– Erroneous information in databases– Misinterpretation of database information (similar

names)– Malfunction of embedded systems (fatal errors)

• Effects of computer errors– Inconvenience– Bad business decisions– Fatalities

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Data-Entry or Data-Retrieval Errors

• A computerized system may fail because wrong data entered into it

• A computerized system may fail because people incorrectly interpret data they retrieve

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Disfranchised Voters

• November 2000 general election• Florida disqualified thousands of voters• Reason: People identified as felons• Cause: Incorrect records in voter database• Consequence: May have affected

election’s outcome

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False Arrests

• Sheila Jackson Stossier mistaken for Shirley Jackson– Arrested and spent five days in detention

• Roberto Hernandez mistaken for another Roberto Hernandez– Arrested twice and spent 12 days in jail

• Terry Dean Rogan arrested after someone stole his identity– Arrested five times, three times at gun point

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Position of Privacy Advocates

• Number of records is increasing• More erroneous records more false

arrests• Accuracy of crime records more important

than ever

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Software and Billing Errors

• Assume data correctly fed into computerized system

• System may still fail if there is an error in its programming

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Errors Leading to System Malfunctions

• Qwest sends incorrect bills to cell phone customers ($600/minute)– A bill of more than $57,000 for a customer

• Spelling and grammar error checkers increased errors (University documents)

• BMW on-board computer failure (Thailand finance minister was trapped in his BMW)

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Errors Leading to System Failures

• Temporarily out-of-control Boeing 777 (Malaysian plane over the Indian ocean-autopilot error)

• Japan’s air traffic control system• London International Financial Futures and

Options Exchange

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Analysis: E-Retailer Posts Wrong Price, Refuses to Deliver

• Amazon.com in Britain offered iPaq (handheld computers) for £7 instead of £275

• Orders flooded in• Amazon.com shut down site, refused to

deliver unless customers paid true price• Was Amazon.com wrong to refuse to fill

the orders?

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Notable Software System Failures• Patriot Missile (28 soldiers killed) –

insufficient precision in an floating-point variable (system clock)

• Ariane 5 ($500 million not insured) – Integer overflow error

• AT&T long-distance network (70 million calls couldn’t be made and 60,000 people lost service)

• Direct recording electronic voting machines• Therac-25 (3 patients killed out of 6

patients) SW error resulted in overdoses.

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Direct Recording Electronic Voting Machines

• After problems with 2000 election, Congress passed Help America Vote Act of 2002

• Provided money to states to replace punch card voting systems

• Many states used funds to purchase direct recording electronic (DRE) voting machines

• Brazil and India have run national elections using DRE voting machines exclusively

• In November 2006 1/3 of U.S. voters used DRE voting machines

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Issues with DRE Voting Machines

• Voting irregularities– Failure to record votes– Overcounting votes– Misrecording votes

• Lack of a paper audit trail• Vulnerability to tampering• Source code a trade secret, can’t be examined• Possibility of widespread fraud through malicious

programming

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Moral Responsibility of theTherac-25 Team

• Conditions for moral responsibility– Causal condition: actions (or inactions) caused the

harm– Mental condition

• Actions (or inactions) intended or willed -OR-• Moral agent is careless, reckless, or negligent

• Therac-25 team morally responsible– They constructed the device that caused the harm– They were negligent

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Uses of Computer Simulations

• Simulations replace physical experiments. Why?– Experiment too expensive or time-consuming– Experiment unethical– Experiment impossible (evolution of the universe)

• Examples of uses of simulations:– Model past events (Evolution of the universe)– Understand world around us (Search for Oil)– Predict the future (weather predictions)

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Validating Simulations

• Erroneous Simulation because of :– Bugs in SW– Model uses SW is flawed

• Solutions:– Verification: Does program correctly implement model?– Validation: Does the model accurately represent the real

system?

• Validation methods– Make prediction, wait to see if it comes true (Car crash)– Predict the present from old data (Weather forecast)– Test credibility with experts and decision makers

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Software Engineering

• SE is a four steps process:– Specs: determine the functions to be

performed

– Development: produce SW that meet specs.

– Validation: testing the SW

– Evolution: Modify SW to meet change requirements

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Software Engineering: Specification

• Followed SW Crises in 1960s• Specifications of SE:

– Determine system requirements– Understand constraints– Determine feasibility (Budget and schedule)

• End products– High-level statement of requirements (Summary)– Mock-up of user interface– Low-level requirements statement (Detailed)

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Software Engineering: Development

• Create high-level design• Discover and resolve mistakes, omissions

in specification• CASE tools to support design process• Object-oriented systems have advantages• After detailed design, actual programs

written• Result: working software system

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Software Engineering: Validation (Testing)

• Ensure software satisfies specification• Ensure software meets user’s needs• Challenges to testing software

– Noncontinuous responses to changes in input. No identical data set of inputs

– Exhaustive testing impossible. Infinite number of different inputs

– Testing reveals bugs, but cannot prove none exist

• Test modules, then subsystems, then system

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Software Quality Is Improving

• Standish Group tracks IT projects• Situation in 1994

– 1/3 projects cancelled before completion– 1/2 projects had time and/or cost overruns– 1/6 projects completed on time / on budget

• Situation in 2006– 1/6 projects cancelled– 1/2 projects had time and/or cost overruns– 1/3 projects completed on time / on budget

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Shrinkwrap Warranties

• Some say you accept software “as is”• Some offer 90-day replacement or money-

back guarantee• None accept liability for harm caused by

use of software

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Moral Responsibility of Software Manufacturers

• If vendors were responsible for harmful consequences of defects– Companies would test software more– They would purchase liability insurance– Software would cost more– Start-ups would be affected more than big companies and thus– Less innovation in software industry– Software would be more reliable

• Making vendors responsible for harmful consequences of defects may be wrong

• Consumers should not have to pay for bug fixes

Chapter 7: Computer Reliability

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