Variation and Human Error

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Variation and Human Error
Douglas M. Stewart, Ph.D. The Anderson Schools of
Management University of New Mexico
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Understanding Human Error

• Mistakes vs. Action Slips
• Intention vs. action
• Skill-based activities
• Poka-yokes address action slips
• Action Slips
• Norman (1981) - Activation Trigger Schemata (ATS)
• Reason and Mycielska (1982) - Cognitive Demons
• Sources of Slips (Norman)
• Faulty activation
• Faulty triggering

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Understanding Human Error (cont.)

• Six common mechanisms (Reason 1990)
• Double capture slips
• Reduced intentionality
• Perceptual confusion
• Interference errors
• Omissions following interruptions
• Mistimed checks (omissions, repetitions and
  reversals)
• Addressing Mistakes
• Skill Rule Knowledge framework (SRK)
• (Rasmussen 1983, 1987)
• Generic Error Modeling System (GEMS)
• (Reason 1987, 1990)

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Generic Error Modeling System

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Adapted from Reason, J. Human Error. p. 64.
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Skill-based GEMS Mechanisms

• (seen above in action slips)
• Inattention
• Double-capture slips
• Omission following interruptions
• Reduced intentionality
• Perceptual confusion
• Interference errors
• Over Attention
• Omissions
• Repetitions
• Reversals

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Rule-based GEMS Mechanisms

• General Contributing factors to Rule-based
  errors
• Information overload
• (signs, countersigns, and non-signs)
• Rule strength
• Rule-based error forms
• Distorted Interpretations
• Frequency and Generality
• Availability (Redundancy)
• Selectivity
• Matching bias (including First exception)
• Communications Fault
• Oversimplification
• Single ordering preference
• Insufficient consideration of process in time
• Difficulties with exponential developments

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Rule-based GEMS Mechanisms (cont.)

• Conservation of mental effort
• Rigidity (Mind set)
• Overconfidence
• Problems with cause and effect
• Spurious Correlations
• Feedback delay
• Other encoding deficiencies
• Incomplete coding of problem set
• Complex task not yet fully learned
• Action deficiencies
• Bad Rules

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Knowledge-based GEMS Mechanisms

• Limitations in Cognitive Capabilities
• Workspace limitations
• Out of sight out of mind
• Biased reviewing
• Thinking in causal series not causal nets
• Thematic vagabonding
• Encysting

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How Mistake-Proofing Works
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How Mistake-Proofing Works (cont.)

• Three Mistake-Proofing Strategies
• Mistake detection and correction
• Action based
• Outcome based (warning poka-yokes)
• Limiting functions (control poka-yokes)
• Alter the environmental inputs
• POET
• Real World Mapping
• Visibility
• Natural and artificial constraints
• Standards
• Ecological Interface Design
• Error tolerant systems
• De-coupling the system, Auto-correct features
• Reversible and soft boundaries
• Systems that degrade gracefully

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Conflict between SPC and Poka-Yoke

• Despite the wide acceptance of poka-yoke by
  practitioners, academic research is limited.
• Reasons
• SPC has more history
• Lack of theoretical basis for poka-yoke
• (Why does it work?)
• SPC and poka-yoke are viewed as substitutes
• (Variation paradigm leads to a rejection of
  poka-yoke methods)
• Solution
• Theory - Psychology of human error.
• SPC and poka-yoke are complements.

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Variation and Mistakes

• Seeking support from overlaps in the literature
  and reconciling competing concepts
• First Order Causes of Defects - Variation and
  Mistakes
• Require different approaches
• Shingo as originator of ambiguity
• Origins of ideas (SPC just counts defects)
• Poka-yoke examples to control defects from
  variation
• Machine mistakes
• Variation and discrete events
• Second Order Causes of Defects
• Causes of mistakes and variation
• Complexity (Hinckley and Barkan)
• Culture

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• Cultural Improvement
• Driving out fear
• Incentives
• Feedback
• Priorities

Culture
Complexity

• Mistake Prevention in the Work Environment
  (Complexity Reduction)
• Design for Assembly
• Visible System Dynamics
• Use of Standards
• Ecological Interface Design
• Workload balance
• Highlight Important Cues
• Information Poka-yokes
• Simplified Task Structures
• Facilitate Correction

Mistakes
Mistake and Defect Detection
Forcing Function (Control)
Physical Grouping and Counting Sequencing Info
rmation

• Preventing the Influence of Mistakes
• De-couple the System
• Facilitate Correction

Source Inspection Self-Checks Successive
Checks Joint Inspection
Outcome-Based (Warning)
Detected by Self
Detected by others
Variation

• Variation Reduction
• Design for Manufacturing
• Robust Design
• Design of Experiments
• SPC - Identify Special Causes

• Variation Control
• SPC
• Process capability
• Source Inspection Poka-yokes

First Order (Primary) Causes of Defects
Second Order Causes of Defects