Human Error Rate Prediction Calculator (THERP-Inspired HEP Screening)

What is Human Error Probability (HEP)?

Human Error Probability (HEP) is an estimate of the likelihood that a person will make an error while performing a task under specific conditions (time pressure, fatigue, procedure quality, supervision, etc.).

THERP (Technique for Human Error Rate Prediction) is a structured method from the field of Human Reliability Assessment (HRA) used to quantify human error probabilities and identify improvements.

Important note (accuracy-first): A full THERP study requires detailed task breakdown and formal data tables. This online tool is a screening calculator to help teams quickly identify where human error risk is likely higher and what controls can reduce it.

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Why measure human error risk in safety and operations?

• Prevention: Identifies tasks where mistakes are more likely before incidents occur.
• Prioritization: Helps focus on controls (procedures, training, supervision, verification) where they matter most.
• Learning culture: Shifts conversation from blame to system conditions (fatigue, time pressure, unclear steps).
• Audit readiness: Demonstrates proactive risk assessment aligned with modern safety management practices.

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THERP Screening Calculator (Human Error Probability Estimate)

Select the task type (baseline) and adjust the performance conditions. The calculator produces an estimated HEP% and highlights the biggest drivers.

How to Use This Human Error Calculator (Step-by-Step)

1. Assess the current situation:
   Select the task type and rate all conditions as they exist today for a high-risk activity such as confined space entry, lifting operations, hot work, LOTO, maintenance shutdowns, or emergency response.
2. Calculate the baseline risk:
   Run the calculator to obtain an estimated Human Error Probability (%), representing the relative likelihood of error under current conditions.
3. Simulate risk controls:
   Improve selected parameters—reduce time pressure, add supervision or independent verification, improve procedures, ensure rested manpower—and recalculate.
4. Compare before vs after:
   Review the difference between the results without controls and with controls to see how proposed measures reduce human error likelihood.
5. Make an informed decision:
   Use this comparison to decide whether the task can proceed safely or if additional controls are required before execution.

Note: This tool supports risk-based decision-making and prioritization. For critical or life-threatening activities, a formal Human Reliability / THERP study is recommended.

Task Type (Baseline Error Likelihood): Routine checklist step (simple, familiar) Standard procedure step (normal complexity) Complex task with multiple steps / interfaces Non-routine / abnormal situation response Emergency / high-stress decision under pressure

Baseline is a screening assumption; use your internal data if available.

Total Manhours on Task (optional, for “expected errors” estimate):

If provided, calculator estimates expected errors = HEP × task executions (approx).

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Performance Shaping Factors (PSFs)

Choose the option that best matches current conditions. Each factor adjusts the baseline.

Time Pressure: Low (ample time) Normal High (rushed) Very high (deadline / queue / congestion)

Fatigue / Shift Condition: Rested / normal shift Normal Long shift / moderate fatigue Severe fatigue / night shift overload

Procedure Quality (clarity, step-by-step, available): Excellent (clear, verified, easy to follow) Adequate Weak (confusing / outdated) Poor / missing / informal instructions

Competency / Training: Highly competent / frequent practice Competent New / limited experience Unfamiliar task / skill gap

Supervision / Verification: Independent verification / 2-person check Active supervision Normal Limited supervision No verification / lone work

Work Environment (noise, heat, visibility, congestion): Good environment Normal Challenging Very challenging (heat/noise/poor access)

Result

Your result will appear here

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Worked Example

Scenario: Complex task with multiple steps under high time pressure and limited verification.

• Baseline: Complex task = 0.03
• Time pressure: 1.3×
• Verification limited: 1.25×
• Other factors normal: 1.0×

HEP ≈ 0.03 × (1.3 × 1.25) = 0.04875 ≈ 4.88%

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Important Disclaimer & Professional Support

Human Error Probability (HEP) assessment is not a simple calculation. Formal THERP / Human Reliability Analysis (HRA) requires:

• Detailed task decomposition and step-by-step analysis
• Context-specific human error data and expert judgment
• Evaluation of operational, organizational, and human factors

This online calculator is provided for learning, awareness, and preliminary screening only. It should not be used as a substitute for a detailed, site-specific human reliability study—especially for high-risk activities, critical operations, or regulatory decisions.

If you need a formal Human Error / Human Reliability Assessment (THERP or equivalent) for a specific task, activity, or operation, we recommend engaging qualified professionals.

📩 Contact: info@himpre.com
Himaya Prevention Professionals can support detailed human factors and human error studies tailored to your site, industry, and risk profile.

Typical applications include: critical maintenance activities, confined space work, lifting operations, energy isolation (LOTO), emergency response tasks, control room operations, and other high-risk or non-routine activities.

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FAQ

1) Is this “true THERP”?
No. This is a THERP-inspired screening calculator to quickly prioritize improvements. Formal THERP requires detailed task analysis and data tables.

2) What should I do if HEP is High?
Reduce risk by improving the highest drivers: remove time pressure, strengthen procedures, add verification, improve training, reduce fatigue, and simplify interfaces.

3) Can this be used for audits or legal reporting?
Use it as internal decision-support only. For safety-critical operations, use a documented HRA/THERP study and your organization’s governance process.

4) How can I make this more accurate?
Replace baseline assumptions with your own data, add task-specific PSFs, and calibrate multipliers based on incident history and competency assessments.

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Explore All Safety KPI Calculators by Himaya Prevention

Use these free tools to calculate key HSE indicators and benchmark your site performance.

Human Error Rate Prediction Calculator EXCLUSIVE NEW Unsafe Act & Condition Rate NEW Near Miss Frequency Rate (NMFR) NEW DART Rate Calculator NEW TRIFR Calculator Lost Time Injury Severity Rate (LTISR) Fatal Accident Frequency Rate (FAFR) First-Aid Injury Frequency Rate (FAIFR) Lost Time Injury Frequency Rate (LTIFR)

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References

• Swain & Guttmann (1983), Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications (NUREG/CR-1278)
• Overview and background of THERP and HRA methods
• Review literature on THERP/HRA evolution and limitations

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