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Sampling Strategy Calculator

Minimum sample size and SEG characterisation adequacy per AIHA IHSS and NIOSH methods

How to use
Enter the assumed geometric standard deviation (GSD) of the exposure distribution, the target decision point as a fraction of the OEL, and the required statistical confidence. Press Calculate to determine the minimum number of samples needed and to generate the operating characteristic (OC) curve for the SEG.
SEG Characteristics
GSD describes the spread of exposures within the SEG. Typical values: 1.5–2.0 (homogeneous process), 2.0–3.0 (variable task mix), 3.0–5.0 (highly variable). GSD cannot be less than 1.
GSD Presets
What fraction of exposures above the decision point is acceptable?
Statistical confidence
Existing Data (optional)
If you already have samples from this SEG, enter the details below to assess whether your current dataset is sufficient.
About this reference
This tab provides standard guidance tables for SEG characterisation. Values are drawn from AIHA A Strategy for Assessing and Managing Occupational Exposures (4th ed.), NIOSH Manual of Analytical Methods, and Leidel, Busch & Lynch (1977).
AIHA Risk Band and Sample Size Guidance
Risk Band 95th %ile / OEL Min n recommended Rationale
Band 1
Very highly controlled		 < 1% 1–3
Qualitative only		 Exposure so low that sampling mainly confirms absence; quantitative characterisation is not cost-effective.
Band 2
Well controlled		 1–10% 3–6 Sufficient to confirm low exposure; not designed for precise characterisation of the distribution.
Band 3
Action level		 10–50% 6–10 Adequate characterisation for trend monitoring; formal UCL calculations are feasible.
Band 4
Near OEL		 50–100% ≥ 10 Formal characterisation required; sufficient samples needed to resolve whether the 95th percentile exceeds the OEL.
Band 5
Exceeds OEL		 > 100% ≥ 6
Immediate control; sampling to verify		 Controls must be implemented immediately. Further sampling confirms effectiveness of controls, not initial characterisation.
GSD Guidance by Work Pattern
Work pattern Typical GSD range Example
Single substance, steady process, homogeneous SEG 1.3–1.8 Constant solvent cleaning operation
Mixed tasks, some variability 1.8–2.5 General manufacturing
Intermittent tasks with high peaks 2.5–3.5 Spray painting, welding
Highly variable or complex SEG 3.5–5.0 Mining, construction, agriculture
AIHA 3-Zone Decision Rule
The AIHA framework evaluates compliance based on the upper and lower confidence limits on the 95th percentile of the exposure distribution. Three decision zones are possible after collecting sufficient data.
Zone Decision criterion Action
Accept UCL on 95th percentile < OEL Conclude compliant. Document and periodically re-survey to verify conditions have not changed.
Reject LCL on 95th percentile > OEL Conclude non-compliant. Implement controls immediately. Re-survey after controls are in place.
Indeterminate Confidence interval straddles OEL Collect more data, apply the precautionary principle, or implement interim controls and re-survey.
Sources: AIHA A Strategy for Assessing and Managing Occupational Exposures (4th ed., Mulhausen & Damiano); NIOSH Manual of Analytical Methods; Leidel, Busch & Lynch (1977) Occupational Exposure Sampling Strategy Manual (NIOSH Publication 77-173); Tuggle (1982).