A brief Introduction for THERP.

1. TECHNIQUE FOR HUMAN ERROR
PREDICTION (THERP)
Presented by:
Anand Kumar
11RE60R04
M.Tech(2nd YR)
Reliability Engineering

2. CONTENTS
• Introduction
• Steps of THERP
• Outline of THERP for HRA
• Advantages and Limitations
• Discussion on Data Problem
• Sources of Human Performances Estimates
• A Brief Overview of HEPs Tables

3. THERP
• Technique For Human Error Prediction (THERP): A method to
predict human error probabilities and to evaluate the degradation of
a man machine system likely to be caused by human errors alone or
in connection with equipment functioning, operational procedures
and practices, or other system and human characteristics that
influence system behaviour.
• Detailed task analysis
• Emphasis on critical procedural steps
• Development of THERP trees
– Structured logic for task evaluation
– Documentation of analysis
– Facilitates computation of Human Error Probabilities

4. THERP (continued)
• Error rates primarily from Experts’ judgment
• Performance-shaping factors: factor that influences
performance depending on task and domain
– Task complexity
– Available personnel
– Training
– Available time
– Dependence among personnel
– Dependence among tasks
• Use of screening values to control scope of analysis

5. STEPS OF THERP
(1) Define the system failures of interest.
(2) List and analyze the related Human operations.
(3) Estimate the relevant error probabilities.
(4) Estimate the effects of Human errors on the system failure
events.
(5) Recommend changes to the system and recalculate the system
failure probabilities.

6. OUTLINE OF THERP PROCEDURE FOR HRA

7. ADVANTAGES OF THERP
• Easy to use
• Modest cost
• Tabulated values reduce need for Analyst judgment
• Selection rules reduce variability in results
• No specialized software needed

8. LIMITATIONS OF THERP
• Excessive emphasis on procedural details
• Detailed models mask true causes for errors
• False confidence that analyses are very precise
• No structured input from Plant Personnel
• Limited use for understanding important issues and
recommending improvements

9. DATA PROBLEM FOR HRA
• Scarcity of data on Human Performance that can be
used to estimate HEPs for tasks
• Error probabilities for many tasks are very small
• Penalty associated with the error commission, which
discourages the identification of individuals who
commit the errors
• Administrative problems and costs of recording and
analyzing errors

10. SOURCES OF HUMAN PERFORMANCE ESTIMATES
• Extrapolation of data from sources combined with
expert judgment
• Use of Expert judgment only
• Measurements of actual or simulated task times
• Combination of the estimates of those who perform
the tasks
• Collate errors across individuals and record error
relative frequencies and performance times by task
and other descriptors.

11. TABLES OF ESTIMATED HUMAN ERROR PROBABILITIES
• Summarizes the estimated Human Error
Probabilities (HEPs) and their Error Factors (EFs)
• Tables Based on
1. Field studies and experiments in Industrial
settings
2. Experiments Using Artificial Tasks
3. Use of Expert Opinion

12. QUICK REFERENCE GUIDE

13. SEARCH SCHEME FOR USE OF TABLES

14. SEARCH SCHEME FOR USE OF TABLES

15. SEARCH SCHEME FOR USE OF TABLES

16. THANK YOU !