:"For the 1993 hip-hop single by the Wu-Tang Clan, see C.R.E.A.M."

CREAM is an acronym for Cognitive Reliability Error Analysis Method, a Human reliability analysis technique developed by Erik Hollnagel. It is a bi-directional analysis method, meant to be used for both performance prediction and accident analysis. Unlike first generation error analysis methods like THERP, CREAM represents a second generation tool allowing for better analysis by abandoning the hierarchical structure of previous methods and providing better separation between objective and subjective error.

CREAM 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.

CREAM is a second generation HRA method. However compared to many other such methods, it takes a very different approach to modelling human reliability. There are two versions of the technique, the basic and the extended version, both of which have in common two primary features; ability to identify the importance of human performance in a given context and a helpful cognitive model and associated framework, usable for both prospective and retrospective analysis. Prospective analysis allows likely human errors to be identified while retrospective analysis quantifies errors that have already occurred.

The concept of cognition is included in the model through use of four basic ‘control modes’ which identify differing levels of control that an operator has in a given context and the characteristics which highlight the occurrence of distinct conditions. The control modes which may occur are as follows:

* Scrambled control: the choice of the forthcoming action is unpredictable or haphazard. The situation in question may be portraying rapid alterations in unexpected ways thus eliminating the operator’s ability or opportunity to make deductions about the next action required.
* Opportunistic control: the next action is determined by superficial characteristics of the situation, possibly through habit or similarity matching. The situation is characterised by lack of planning and this may possibly be due to the lack of available time.
* Tactical control: performance typically follows planned procedures while some ad hoc deviations are still possible.
* Strategic control: plentiful time is available to consider actions to be taken in the light of wider objectives to be fulfilled and within the given context.

The particular control mode determines the level of reliability that can be expected in a particular setting and this is in turn determined by the collective characteristics of the relevant Common Performance Conditions (CPCs).


CREAM was developed by Eric Hollnagel in 1998 following an analysis of the methods for HRA already in place. It is the most widely utilised second generation HRA technique and is based on 3 primary areas of work; task analysis, opportunities for reducing errors and possibility to consider human performance with regards to overall safety of a system.

The aim of utilising this methodology is to assist an analyst in four main areas:
* identify work, actions or tasks within the system which necessitate or essentially depend on human thinking and which are therefore vulnerable to variations in their level of reliability.
* identify the surrounding conditions in which the cognition of these situations may be reduced and therefore determine what actions may lead to a probable risk
* compile an evaluation from the assessment of the various outcomes of human performance and their effect on system safety – this can then be utilised as part of the Probability Risk assessment (PRA).
* make suggestions as to how identified error producing conditions may be improved and therefore of how the system’s reliability can be enhanced whilst also reducing risk.


"1. Task Analysis"

The basic method adopted by the CREAM technique provides an immediate reliability interval based on an assessment of the given control mode, as highlighted by the figures provided in the table below. As can be seen by the contents of the table, each of the specified control modes has an individual reliability level. In the extended CREAM version, the control modes play the role of a weighting factor which scales a nominal failure probability associated to a given cognitive function failure. This version of CREAM is intended to be used for the purposes of a more in depth analysis of human interactions.


In order to calculate the combined CPC score, the assigned ratings of the CPCs are entered in the table as shown in step 3 of the methodology section. Using certain rules [19] an assessment is made as to whether it is necessary to adjust the CPCs. In this example this is not necessary. Therefore the combined CPC score for this example is [3,5,1] . This is interpreted as the CPC’s pointing to a reduced performance reliability, 4 CPC’s indicate that there is no significant influence and 1 CPC suggests an improved performance reliability.


By determining the most likely control mode for the example, the general action failure probability can also thus be identified. Referring to the graphical display in Figure 1, the result for this example is that the operator is expected to be in an opportunistic control mode. This adequately relates to the assumption provided earlier that the operator under consideration has only slight experience or training for the task and there is insufficient support for the operations involved in the task. It may therefore be suggested that the operator may task a ‘try and test’ approach, particularly for complicated tasks such as increasing the temperature under controlled conditions.

The last stage of the process is to determine the probability interval for the expected control mode, the opportunistic control mode. Referring to table 1, for this example, the general action failure probability is within the range of 1.0 E-2 < p < 0.5 E-0. As this is not regarded as an acceptable there is no clear and justified reason to continue with the analysis being undertaken.


* the technique allows for the direct quantification of human error probability (HEP)

* it also allows the assessor using the CREAM method to specifically tailor the use of the technique to the contextual situation [2]

* the resultant model is highly integrate-able into the primary safety process in use

* the technique uses the same principles for retrospective and predictive analyses [2]

* the approach is very concise, well structured and follows a well laid out system of procedure [2]


* the technique requires a high level of resource use, including lengthy time periods for completion [2]

* CREAM also requires an initial expertise in the field of Human factors (HF) in order to use the technique successfully and may therefore appear rather complex for an inexperienced user [2]

* CREAM does not put forth potential means by which the identified errors can be reduced [2]

* The time required for application is very lengthy


[1] Hollnagel, E. (1998) Cognitive Reliability and Error Analysis Method – CREAM. Elsevier Science, Oxford.

[2] Salmon, P., Stanton, N.A., & Walker, G., 2003. Humans Factors Design Methods Review. Defence Technology Centre.

[3] XuHong, He. (2005). A simplified CREAM prospective quantification process and its application. Reliability Engineering & Safety System. 93, 298-306.

[4] Kim, I.S., (2001). Human Reliability Analysis design review, Annals of NuclearEnergy, 28, 1069-1081.

External links

* [ CREAM - Cognitive Reliability and Error Analysis Method]
* [ CREAM Navigator - a web-based software tool]

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