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Fatigue and Risk Index

Fatigue and Risk indexes in Skillo​

We calculate the Fatigue Index and Risk Index for each shift.

We use the same algorithm that was in the rr446cal.xls file from Network Rail.

We did not and do not alter the algorithm in any way.

The Fatigue and Risk index algorithm​

The HSE FRI is an advanced tool designed to calculate the Fatigue Index and Risk Index of employee shift schedules.

Unlike basic calculators that simply look at the number of hours an employee works or how many hours of rest they get, this algorithm acts as a biomathematical model.

It simulates the human biological clock to understand how exhausted a worker will actually feel.

The Biological Clock and Circadian Rhythm​

The most crucial feature of this algorithm is how it handles the Circadian Rhythm (our body's internal 24-hour clock). Human biology dictates that we are meant to be awake during the day and asleep at night.

Because of this, the algorithm heavily penalizes schedules that force a worker to fight their biology. For example, if an employee switches from a day shift to a night shift, their Fatigue and Risk scores will spike dramatically—even if they are given a perfectly legal, long break between the shifts.

This happens for two reasons:

  1. Truncated Recovery: If a night-shift worker goes to bed at 7:00 AM, their biological clock will wake them up after only a few hours of sleep, meaning they cannot fully recover. The algorithm recognizes this and calculates a "sleep debt".
  2. The "Witching Hour": Working during the natural circadian low (typically between 2:00 AM and 5:00 AM) mathematically triggers an automatic increase in risk and alertness failure within the algorithm, regardless of how rested the worker is.

In short, this algorithm measures not just HOW MUCH time off a worker gets, but WHEN that time off occurs.

What we send to the algorithm​

We send in the same information that you would put in the excel file.

There are 4 parameters for the algorithm describing the shift break pattern.

We did not want to create as much friction in Skillo so we simplified things a bit.

Described below.

🔵 day

  • The day the shift starts on

🔵 on duty

  • Shift start time

🔵 off duty

  • Shift end time

🔵 commuting time

  • The total of the departure plus arrival time divided by 2

🔵 breaks frequency

  • The total duration of the shift, minus the break, divided by 2.

  • If you have a 9 hrs shift with a 1hr break this will be (9 - 1) / 2 = 4.

🔵 breaks average

  • The break duration.

🔵 longest work

  • The total duration of the shift, minus the break, divided by 2.

  • If you have a 9 hrs shift with a 1hr break this will be (9 - 1) / 2 = 4.

🔵 breaks after longest work

  • The break duration.

🔵 workload

  • One of the following:

    • Extremely demanding, no spare capacity

    • Moderately demanding, little spare capacity

    • Moderately undemanding, some spare capacity

    • Extremely undemanding, lots of spare capacity

🔵 attention frequency

  • One of the following:

    • All or nearly all of the time

    • Most of the time

    • Some of the time

    • Rarely or nearly none of the time

For any enquiries or suggestions on this topic please send an email to team@skillo.co.uk.

Thank you!