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RR446 - The development of a fatigue / risk index for shiftworkers

This report describes the work carried out to revise and update the HSE Fatigue Index (FI). Extensive changes have been made to the previous version, incorporating recent information relating to a variety of issues including cumulative fatigue, time of day, shift length, the effect of breaks and the recovery from a sequence of shifts. In addition, a review has been carried out of trends in risk related to shift work, and this has enabled the final version to incorporate two separate indices, one related to fatigue (the Fatigue Index) and the other to risk (the Risk Index). While the two indices are similar in many respects they diverge in others. The main differences are due to the different trends with respect to time of day in fatigue and risk. The index has been implemented in the form of a spreadsheet, the design of which has incorporated feedback from users of the previous index.

This spreadsheet has now been updated (January 2013). The new version of the FRI (v2.3) allows the spreadsheet macro to run in MS Excel 2007 or MS Excel 2010 without errors and to generate the required graphs and diagrams, but has not been otherwise amended.

Interpretation of fatigue and risk index outputs

The Fatigue and Risk Index (FRI) was designed primarily for comparing different shift schedules but can also be used to identify any particular shift, within a given schedule, which may be of concern.

Whilst the FRI is a useful tool, which can be used to help assess the risks of fatigue and injury, it should not be relied upon as the sole or primary means of assessing these risks.

Shift work planners should always start by considering the guidelines in Managing shift work: Health and Safety Guidance (HSG 256), which includes background information on the health and safety risks associated with shift work and fatigue, UK legal duties and practical guidance on how to reduce the risks. FRI outputs should also be considered in conjunction with feedback from staff on how tiring they find their work patterns.

Interpretation of FRI fatigue scores

When interpreting fatigue scores it is important that the scores for each shift are considered rather than the average scores over a schedule.

Fatigue scores from 0 to 100 represent the average probability expressed as a percentage of high levels of sleepiness. For example, a score of 50 on the Fatigue Index represents a 50% chance of employees achieving a Karolinska Sleepiness Score (KSS) of 8 or 9. The KSS is a nine-point scale ranging from one (extremely alert) to nine (extremely sleepy – fighting sleep). It has been extensively validated and high scores are known to be associated with a high frequency of micro sleeps. In other words, if a score of 50 is obtained for a particular shift then the FRI represents a 50% chance that workers will experience fatigue to the extent that they may struggle to stay awake on that particular shift.

HSE does not currently specify threshold scores for fatigue. Recent research by the Health and Safety Laboratory (HSL) for the UK Office of Rail and Road (ORR) surveyed shift patterns in the UK rail sector. It found that the great majority of working patterns surveyed produced fatigue scores less than 30 - 35 for day shifts and 40 - 45 for night shifts. These values reflect what was found to be achievable by the great majority of the rail companies surveyed at the time (2008), rather than ‘good practice’ in managing fatigue. Organisations should not assume that just because FRI analysis of their working patterns produces FRI values below the 2008 'indicative thresholds' that they need do no more - it will often be reasonably practicable to improve fatigue controls further. 

HSE's view is that organisations should aim to reduce scores to as low a value as reasonably practicable rather than merely attempting to achieve scores below those identified by the HSL/ORR study. Feedback should then be sought from staff about how tiring they find their working patterns.

Interpretation of FRI risk scores

Again it is important to consider the scores for individual shifts rather than the average scores over a schedule.

A risk score of 1 is based on the average level of risk of accident/error attained in studies on people working 12 hour shifts on a 2 day, 2 night, 4 rest day schedule in the rail sector. Consequently, a risk score of 2, could be interpreted as a doubling of risk on that particular shift schedule.

Organisations need to determine the level of risk that they would consider acceptable based on the type of work being done and the individuals doing it. For example, a high risk score might be considered acceptable by an organisation if the work is low risk and is undertaken by highly competent and experienced workers. Alternatively, if the work is safety critical/hazardous and/or undertaken by less competent/inexperienced workers then an organisation may decide that risk scores above a certain level are unacceptable for that particular type of work or particular groups or workers. In general, organisations should try to keep risk scores as low as they can, but the level of risk considered acceptable by an organisation needs to be determined by someone familiar with the type of work being done and the workers undertaking that work.

Limitations of the Fatigue and Risk Index

It is important to note that the FRI is based on group data and does not, take into account factors such as individual differences (eg age), specific work-related issues (eg exposure to chemical hazards) or social factors (eg lifestyle, domestic responsibilities), although it should be recognised that that these may affect a worker's tolerance to shift work. The industry sector and the type of work done will also affect the risk of fatigue and it is important to recognise that the mathematical formulae used in the FRI cannot take into account such variations.

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Updated 2014-08-27