The Technique for Human Error Rate Prediction (THERP) is a foundational method used in Human Reliability Assessment (HRA) to identify and quantify the likelihood of human errors in safety-critical tasks. It is widely referenced in process safety, high-risk operations, and reliability engineering where human actions can significantly influence outcomes.

Human Error Analysis focuses on recognizing where errors may occur, understanding why they occur, and estimating how those errors can contribute to incidents, near misses, and major accidents. THERP supports this by providing a structured approach to task breakdown and probability estimation.

Background and History

THERP originated from safety and reliability research in the 1970s, including work associated with Sandia National Laboratories (USA), to support quantitative assessments in complex, high-hazard systems. It was designed for environments where tasks can be described step-by-step and where error likelihood can be estimated using tabulated data and structured rules.

The core idea is straightforward: while it may be difficult to predict rare, non-routine behavior with precision, it is possible to analyze defined and repeatable actions and estimate their error probability using evidence, expert judgment, and performance-shaping factors.

Key Concept: Basic Error Rate (BER)

For selected tasks, THERP assigns a Basic Error Rate (BER), representing the nominal probability that a given human action will be performed incorrectly. BER values are commonly expressed as an error probability per action (and are often communicated using “per million operations” language in reliability contexts). In practice, BER is then adjusted based on operational conditions such as complexity, time pressure, stress, supervision, interface design, and procedure quality.

Technique for Human Error Rate Prediction (THERP)

Human error probability data is of particular interest to safety specialists because it can be used as an input to Fault Tree Analysis (FTA) and Event Tree Analysis (ETA). These methods help quantify how combinations of equipment failures and human actions can lead to undesired outcomes.

Where THERP Is Used (Practical Examples)

  • Energy isolation and verification tasks (LOTO), including independent checks
  • Permit-to-Work execution: confined space entry, hot work, line breaking
  • Lifting operations: rigging checks, critical lift verification, exclusion zones
  • Maintenance shutdowns and complex troubleshooting activities
  • Control room operations and abnormal situation response
  • Emergency response actions requiring rapid decision-making

Steps of THERP

  1. Define the system failure (or undesired event) of interest.
  2. List and analyze the related human operations (task breakdown into steps).
  3. Estimate the relevant error probabilities for each step (BER and adjustments).
  4. Estimate the effects of human errors on system failure events.
  5. Recommend changes (controls) and recalculate the failure probabilities.

Advantages of THERP

  1. Structured and repeatable approach
  2. Modest cost for a screening-level assessment
  3. Tabulated values can reduce variability in analyst judgment
  4. Selection rules support consistency of application
  5. Does not inherently require specialized software

Limitations of THERP

  • Can over-emphasize procedural detail and miss underlying human factors
  • Detailed models may mask the true drivers of error (culture, fatigue, design)
  • Numbers can create false confidence if context is not understood
  • Quality improves significantly when plant/site personnel input is structured
  • Not ideal as a standalone method for “why” errors occur without supporting HF analysis

Quick FAQ

Is THERP only for nuclear or process safety?
No. While it has roots in high-hazard industries, THERP concepts apply to any safety-critical task where human actions influence outcomes.

Does THERP replace risk assessment?
No. THERP complements risk assessment by quantifying the contribution of human error within a broader hazard analysis and control framework.

Is THERP a simple calculation?
No. A proper study requires task analysis, assumptions, context, and competent interpretation. Simplified tools can be useful for learning and screening, but not for final safety decisions.

Try the THERP Screening Calculator

Explore how task conditions (time pressure, fatigue, procedures, supervision, environment) can influence human error probability.

Human Error Rate Prediction Calculator

Note: Human error probability assessment is not a simple calculation. The calculator is intended for awareness and example purposes only. For a formal THERP / HRA study for a specific high-risk activity or operation, contact info@himpre.com (Himaya Prevention Professionals).