Fatigue Index vs Risk Index (FI vs RI)

In the HSE RR446 fatigue context, FI is the Fatigue Index and RI is the Risk Index. They are two related outputs from fatigue risk index (FRI) methods, calculated from the same roster inputs.

Both use the same roster inputs — shift times, duration, rest between duties, breaks, and cumulative work history — but they answer different questions. They can diverge, especially around time-of-day effects.

Neither FI nor RI is a legal limit, a fitness-for-duty test, or proof that a shift is safe or unsafe. They are modelled, decision-support outputs that should be reviewed by competent persons alongside wider fatigue risk assessment and operational judgement.

Fatigue Index (FI) — modelled sleepiness likelihood

The Fatigue Index is typically expressed on a scale from 0 to 100.

In the RR446 research, FI is a modelled estimate of the probability that a worker would experience very high sleepiness during a duty period. That formulation is linked to levels 8–9 on the Karolinska Sleepiness Scale (KSS). ORR Appendix D paraphrases the same index as the estimated probability that a person working the pattern will feel very fatigued at some point during the shift. Those are related public explanations, not identical quotations.

Plain-English reading:

Higher FI → the model predicts greater likelihood of severe sleepiness during that shift
Lower FI → the model predicts lower likelihood of severe sleepiness for the pattern
FI describes acute fatigue likelihood for the duty — not whether a specific person feels tired today

FI does not measure real-time alertness. It does not predict whether a particular accident will occur.

Example framing (illustrative)

If FI = 50 for a shift, the model represents roughly a 50% probability of very high sleepiness for that pattern — not certainty about any individual worker on any given day.

Risk Index (RI) — relative risk comparison

The Risk Index is typically expressed as a relative multiplier, with a reference schedule often normalised around 1.0.

In the RR446 research, RI compares modelled accident/incident risk for a duty or pattern against a reference schedule. ORR Appendix D describes that reference as 12-hour shifts on a typical two-day, two-night, four-off schedule, and treats RI as a relative comparison rather than an acceptable/unacceptable decision.

Plain-English reading:

RI = 1.0 → broadly comparable to the reference pattern in the model
RI = 1.5 → model suggests roughly 50% higher relative risk than the reference
RI = 2.0 → model suggests roughly double the relative risk

RI is comparative. It is most useful when comparing roster options or identifying shifts that stand out within a schedule. It is not an absolute probability of an accident on a given day.

RI does not prove a shift is dangerous. It does not replace management judgement about task risk, worker competence, or controls in place.

Why FI and RI can diverge

Fatigue and accident risk do not always peak at the same time of day. HSE’s framework reflects this:

Aspect	Fatigue Index (FI)	Risk Index (RI)
Primary question	How likely is very high sleepiness during the duty?	How does modelled relative accident risk compare to a reference?
Scale	Typically 0–100 (probability-like)	Relative multiplier (reference ≈ 1.0)
Time-of-day sensitivity	Strong — circadian sleepiness effects	Strong — but risk curve differs from fatigue curve
Best used for	Comparing sleepiness exposure across patterns	Comparing relative risk across patterns
Common mistake	Treating FI as fitness for duty	Treating RI as accident prediction

HSE’s calculator guidance recommends reviewing both indices when assessing a pattern — and examining individual shifts, not only schedule averages. That historic calculator was later withdrawn from the HSE website; the FI and RI concepts remain research outputs, not a currently hosted HSE tool.

Three components behind both scores

FI and RI draw on the same underlying schedule structure described in RR446:

Component	What it captures
Cumulative	Fatigue build-up across consecutive shifts and insufficient recovery
Duty timing	Start time, shift length, and circadian (time-of-day) effects
Job / breaks	Workload, vigilance demand, and break patterns within the shift

A single shift is usually assessed in the context of the whole schedule — the cumulative component needs prior shifts and rest days to be meaningful.

Policy thresholds are not HSE mandates

Many organisations apply colour bands or trigger levels to FI and RI. These are organisational policy thresholds — not universal legal limits.

RR446 does not itself establish universal mandatory FI/RI pass/fail values. ORR Appendix D records historic rail indicative values and cautions against treating them as satisfactory/unsatisfactory verdicts. Organisations should aim to reduce scores so far as is reasonably practicable, seek feedback from staff on how tiring patterns feel, and not treat any historical survey benchmark as a ceiling on improvement.

Contractual fatigue standards on specific infrastructure or projects may define response levels — those apply to organisations subject to those requirements, not to all UK employers.

How to use FI and RI responsibly

Sensible practice:

Compare roster options at design stage — not only inspect one pattern in isolation
Review shift-by-shift — averages can hide high-risk individual duties
Read both FI and RI — they emphasise different aspects of exposure
Document assumptions, scores, and who reviewed them
Combine with worker feedback, supervision, and wider FRMS controls
Reassess when actual hours, travel, or assignments change

See limitations of fatigue models for population averages, input quality, and individual variation.

What this page does not cover

This page explains FI and RI concepts from HSE RR446-based methods. It does not reproduce proprietary standard thresholds, endorse specific software, or state organisation-specific trigger levels. It does not address fitness for duty or legal working time limits.

References

RR446 report — UK Government Web Archive copy — HSE research report (2006); archived copy, not hosted by HSE
HSE research report series: 401–500 — includes RR446 — official HSE listing; not the RR446 PDF
Fatigue and risk index calculator guide — UK Government Web Archive copy — historic HSE calculator guide; not a currently maintained HSE page
Appendix D: Benefits and limitations of fatigue risk assessment tools — ORR — ORR paraphrase of FI/RI and historic rail indicative values
Fatigue — HSE — fatigue as a workplace hazard; tools may support wider risk assessment
Managing shift work (HSG256) — shift work assessment context for using quantitative tools