Success likelihood index method

Success Likelihood Index Method (SLIM) is a technique used in the field of Human reliability Assessment (HRA), for the purposes of evaluating the probability of a human error occurring throughout the completion of a specific task. From such analyses measures can then be taken to reduce the likelihood of errors occurring within a system and therefore lead to an improvement in the overall levels of safety. There exist three primary reasons for conducting an HRA; error identification, error quantification and error reduction. As there exist a number of techniques used for such purposes, they can be split into one of two classifications; first generation techniques and second generation techniques. First generation techniques work on the basis of the simple dichotomy of ‘fits/doesn’t fit’ in the matching of the error situation in context with related error identification and quantification and second generation techniques are more theory based in their assessment and quantification of errors. ‘HRA techniques have been utilised in a range of industries including healthcare, engineering, nuclear, transportation and business sector; each technique has varying uses within different disciplines.
SLIM is a decision-analytic approach to HRA which uses expert judgement to quantify Performance Shaping Factors (PSFs); factors concerning the individuals, environment or task, which have the potential to either positively or negatively affect performance e.g. available task time. Such factors are used to derive a Success Likelihood Index (SLI), a form of preference index, which is calibrated against existing data to derive a final Human Error Probability (HEP). The PSF’s which require to be considered are chosen by experts and are namely those factors which are regarded as most significant in relation to the context in question.
The technique consists of two modules: MAUD (multi-attribute utility decomposition) which scales the relative success likelihood in performing a range of tasks, given the PSFs probable to affect human performance; and SARAH (Systematic Approach to the Reliability Assessment of Humans) which calibrates these success scores with tasks with known HEP values, to provide an overall figure.
Background.
SLIM was developed by Embrey et al. [1] for use within the US nuclear industry. By use of this method, relative success likelihoods are established for a range of tasks, and then calibrated using a logarithmic transformation.
SLIM methodology.
The SLIM methodology breaks down into ten steps of which steps 1-7 are involved in SLIM-MAUD and 8-10 are SLIM-SARAH.
Worked example.
The following example provides a good illustration of how the SLIM methodology is used in practice in the field of HRA.
Context.
In this context an operator is responsible for the task of de-coupling a filling hose from a chemical road tanker. There exists the possibility that the operator may forget to close a valve located upstream of the filling hose, which is a crucial part of the procedure; if overlooked, this could result in adverse consequences, of greater effect to the operator in control. The primary human error of concern in this situation is ‘failure to close V0204 prior to decoupling filling hose’. The decoupling operation required to be conducted is a fairly easy task to carry out and does not require to be completed in conjunction with any further tasks; therefore is failure occurs it will have a catastrophic impact as opposed to displaying effects in a gradual manner.
Required inputs.
This technique also requires an ‘expert panel’ to carry out the HRA; the panel would be made up of for example two operators possessing approximately 10 years experience of the system, a human factors analyst and a reliability analyst who has knowledge of the system and possesses a degree of experience of operation. 
The panel of experts is requested to determine a set of PSFs which are applicable to the task in question within the context of the wider system; of these, the experts are then required to propose those PSFs, of the identified, which are the most important in the circumstances of the scenario. 
For this example, it is assumed that the panel put forth 5 main PSFs for consideration, which are believed to have the greatest effect on human performance of the task: training, procedures, feedback, perceived risk and time pressure.
PSF rating.
Considering the situation within the context of the task under assessment, the panel are asked to provide further possible human errors which may occur that have the potential of affecting performance e.g. mis-setting or ignoring an alarm. For each of these, the experts are required to establish the degree to which each is either optimal or sub-optimal for the task under assessment, working on a scale from 1 to 9, with the latter being the optimal rating. For the 3 human errors which have been identified, the ratings decided for each are provided below:
PSF weighting.
Were each of the identified human errors of equal importance, it would then be possible to obtain the summation of each row of ratings and come to the conclusion that the row with the lowest total rating- in this case it would be alarm mis-set- was the most probable to occur. In this context, as is most often the case, the experts are in agreement that the PSFs given above are not of equal weighting. Perceived risk and feedback are deemed to be of greatest importance, twice as much as training and procedures, which these two are considered to be one and a half times more important than the factor of time. The time factor is of considered of minimal importance in this context as the task is routine and is therefore not limited by time.
The importance of each factor can be observed through the allocated weighting, as provided below. Note that they have been normalised to sum to unity.
Using the figures for the scaled weighting of the PSFs and the weighting of their importance, it is now possible to calculate the Success Likelihood Index (SLI) for the task under assessment.
From the results of the calculations, as the SLI for ‘alarm mis-set’ is the lowest, this suggests that this is the most probable error to occur throughout the completion of the task. 
However these SLI figures are not yet in the form of probabilities; they are only indications as to the likelihood by which the various errors may occur. The SLIs determine the order in which the errors are most probable to occur; they do not delineate the absolute probabilities of the PSFs. To convert the SLIs to HEPs, the SLI figures require to first be standardised; this can be done using the following formulation. 
Result.
If the two tasks for which the HEPs are known are incorporated in the task set which is undergoing quantification then the equation parameters can be determined by using the method of simultaneous equations; using the result of this the unknown HEP values can thus be quantified. In the example provided, were two additional tasks to be assessed e.g. A and B, which had HEP values of 0.5 and 10 -4 respectively and SLIs respectively of 4.00 and 6.00, respectively, then the formulation would be:
The final HEP values would thus be determined as
References.
[1] EMBREY, D.E., Humphreys, P.C., rRosa, E.A., Kirwan, B. & Rea, K., SLIM-MAUD: An approach to assessing human error probabilities using structured expert judgement. NUREG/CR-3518. 1984, US Nuclear Regulatory Commission: Washington DC.
[2] Humphreys, P. (1995) Human Reliability Assessor’s Guide. Human Factors in Reliability Group.
[3] Kirwan, B. (1994). A Practical Guide to Human Reliability Assessment. CPC Press.
[4] Corlett, E.N., & Wilson, J.R. (1995). Evaluation of Human Work: A Practical Ergonomics Methodology. Taylor & Francis.