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Human Error, Safety and Reliability The interaction of design and human capabilities

The Crash of Eastern Flight 401 - Dec. 1972 Account drawn from Danaher (1980) Diverted from approach to Miami Int'l Airport due to light indicating a  malfunction in nose landing gear light. Set autopilot to 2000 feet to reduce work load while checking nose landing gear. Autopilot was inadvertently switched off by pilot, leading to a gradual descent. Crew did not notice descent

The Crash - continued ATC saw plane reading at 900 feet.  The current system could report errors for up to three sweeps. The controller did contact plane but was told all OK. Controller’s attention was diverted by 5 other planes he was responsible for 30 seconds later place crashed killing 99 out of 176.

Crash of Eastern Flight 401 - Errors Pilot Error: not watching altitude which is pilots responsibility Pilot assumed autopilot worked. Controller Error: did not report low altitude to the pilot  (They are required to now). Name all the factors that contributed to this crash?

Error DEFINITION:  an action or lack of action that violates some tolerance limit(s) of the system. Thus defined in terms of system requirements and capabilities. The the occurrence of an error does not imply anything about human, even if it is “the persons fault.” It could be a system flaw

Try This:  Name the Colors red blue yellow green yellow green yellow red blue red green blue yellow yellow blue green green red red yellow blue blue green red red green blue yellow

Try This: Name the Colors

Human Error Probability Error Probability (EP) also known as Human Error Probability (HEP): EP = (# of errors)/(total # of opportunities for the error) value between 0 and 1 gives rate of errors this is a probabilistic value it does not indicate if an error will or will not occur just the likelihood does not indicate type or cause of error

Reliability DEFINITION:  Probability of a successful outcome of the system or component. Reliability is also defined in terms of system requirements. Thus, to evaluate a system it is necessary to know the goals and purposes of the system. Reliability is a probabilistic term. Never seen the perfect system. Calculation of Reliability R = (# of successful operations)/(total # of operations) R = 1 - EP

Human Error Classification Systems - 1 Basic Error Types Unintentional vs. Intentional e.g. mistake on a test vs. what speeds most of us drive. Unrecovered vs. Recovered Recovered:  Error with possibility for damage but no damage actually occurred. (Driving home drunk safely). Unrecovered:  Error where damage could not be avoided. The recovered error of one day could be the next day's unrecovered error.

Human Error Classification Systems - 2 Swain and Guttman’s (1980) Human Error Categories. Error of Omission tpographicl errrs Error of Commission Hitting thumb with the hammer Extraneous Act reading a different class's assignment in class Sequential Error My usual: light the fire before opening the damper Time Error running a red light

Human Error Classification Systems - 3 Meister’s (1971) Types of Failures Based on where the error originates. Operating error: System is not operated according to intended procedure. Design Error: Designer does not take into account human abilities. Manufacturing Error: System is not built according to design. Installation and Maintenance Errors System is not installed or maintained correctly. Scary how common these are.

Human Error Classification Systems - 4

Human Error Classification Systems - 5 Another Cognitively Based System - Slips vs. Mistakes by Reason and Navon Slips are errors in execution Mistakes are errors in planning an action Lawrence’s (1974) Model with Relative Frequency Failure to perceive a hazard 36% Underestimate a hazard 25% Failure to respond 17% Ineffective response 14 Importance: Different types of errors need different types of actions to prevent.

Error Measurement Variable Error:  errors that differ from trial to trial. Constant Error: errors that are constant from trial to trial. Figure - after Champanis (1951) Constant are easier to predict and thus correct.

Human-Machine and Error Analysis A Brief Overview Some Steps that are part of a complete analysis (Swain & Guttman, 1980) 1. Describe system goals and functions. 2. Describe situation. 3. Describe tasks and jobs. 4. Analyze tasks for where errors are likely. 5. Estimate probability of each error. 6. Estimate probability error is not corrected. 7. Devise means to increase reliability. 8. Repeat steps 4 - 7in light of changes.

Calculation of Human Error Probability There are several techniques, will discuss THERP (Swain, 1963) Start at top with probability of correct/incorrect action. Next act is probability of given the last action. These are conditional probabilities - They are not independent. Sum of partial error probabilities at bottom is overall error probability.

Calculation of Human Error Probability - 2 THERP (Cont.) In the diagram, a capital letter is a correct outcome and a small letter is an erroneous action. The | symbol indicates a conditional probability. Apply to starting a car. K = correct key k = incorrect key S = getting key into ignition s = missing ignition P(S|K) is probability of getting key into ignition, given getting correct key. This is the only correct outcome. P(error) = 1-P(S|K)

Calculation of Human Error Probability - 2 To get probabilities of specific actions, it is common to used tabled values. Example HEPs (Swain and Guttman, 1980) Select wrong control in a group .003 of labeled identical controls Turn control wrong direction .5 under stress when design violates population norm. Failure to recognize an incorrect .01 status of item in front of operator

Effects of System Complexity on Reliability In general reliability goes down as number of components goes up (i.e. as complexity goes up). Components in a Series In a series if any single component fails the whole system fails - the four tires on the car. Rs = R1 * R2 * ... * Rn Examples: All components have reliability of 0.90. n = 1 | Rs = = .90 n = 2 | Rs = .9*.9 = .81 n = 3 | Rs = .9*.9*.9 = .73 n = 10 | Rs = .910 = .35

Effects of Redundancy on Reliability Active Redundancy: Both components operate all the time but only one is needed. Failure occurs only when both fail or (EP1)*(EP2) Thus reliability is:RS = 1 - P(1-RI) Example: Use two components and both components have a reliability of 0.90. In a series| Rs = .9*.9 = .81 (above) Redundant| Rs = 1-(1-.9)2  = .99 Two redundant components in a series| Rs = .99*.99 = .98

Techniques to Improve Reliability HARDWARE KISS (Keep It Simple Stupid). A-10 ~33% unavailable at any one time. F-111D ~66% unavailable Apache Helicopter is similar record to F-111D Make it reliable/Quality Control HUMAN Use human factors knowledge in design - back to Three-Mile Island. Use human as redundant system. Others?

Risk Analysis DEFINITION: An estimation of the consequences associated with particular errors. Includes estimate of probability i.e., risk = p(error)*consequences(error) Can be any sort of risk e.g., loss of life, money, etc. Must estimate significance of these various consequences Used to assist many types of decisions: Estimates of safety Estimates of probable success Types of training to use to help operators not to miss important errors