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Full title

A dependability model for domestic systems


Assistive technology, dependability model, trustworthiness, acceptability, fitness for purpose


Our DIRC work developing technology to support older people has also allowed us to reconsider the notion of dependability within domestic spheres. Traditional dependability theory (for example Laprie 1995) is concerned with ensuring software is concerned with faults, failures and errors and their minimization. These can be framed within the notions of ‘fault prevention’, ‘fault tolerance’, ‘fault removal’, and ‘fault forecasting’, which enable the software designer to trace and prevent undesirable problems. . We accept that as far as systems software design is concerned this notion of dependability has some validity, but it is not sufficient in relation to the home as a socio-technical system. The home is not a standardised, order environment; it is not predictive and peoples’ activities and actions are not predefined. How people relate to and utilise technology is not standardised.

Technology might meet certain standards of compliance, but how it is used is often left to the user to determine. For example, when you buy a telephone, it does not state in the instruction manual that you should not use the phone to mix eggs together, or whilst having a shower. There is an assumption by manufacturers that people ‘know’ how to use technology appropriately, and this is where dependability analysis comes to the fore.
Assistive technology at systems designed to support older or disabled people in their own homes have greater dependability requirements than conventional technologies as people rely on AT, and it becomes internalised and part of their everyday experience.. Through our work on the DIRC and EQUATOR projects we have begun to sketch out an alternate model of dependability that relates to the home. We have called this dynamic model the interdependent model. The model can be represented as follows:

Figure 1: The social dependability model of home systems


In order for a system to be dependable, the user must trust that the system will behave as they expect. We define this attribute to be the equivalent of ‘dependability’ in Laprie’s model. That is, it includes the traditional dependability attributes of availability, reliability, etc. However, we will argue below that these need to be re-interpreted to some extent to take into account the characteristics of domestic systems. We include in this category: Availability and Reliability, Safety, Maintainability,Confidentiality and Integrity.


We have argued above that a system that is not acceptable to users will simply not be used. Therefore, it is essential that system characteristics that affect its acceptability such as the system learnability and aesthetics are considered in the design process. We include in this category: Usability, Learnability, Cost, Compatibility, Efficiency, Responsiveness, Aesthetics and fitness for purpose. Fitness for purpose is taken for granted in most of the dependability literature but socio-technical system failures regularly arise because a computer-based system is not fit for the purpose for which it was designed and users of the system have had to adapt their operational processes to accommodate the system’s inadequacies. When the purpose of a system is to cope with disability, users may simply not have this option and the system may simply be unused.


Within the home both the environment and the users of the systems change. This is particularly true for elderly disabled people whose capabilities tend to decline as they age. Therefore, if system dependability of not to degrade, then it must be able to evolve over time, generally without interventions from the system’s designers. We can identify three types of modification that may be made to domestic systems:

• Addition of new equipment
• System configuration or re-configuration by its users
• Configuration or re-configuration of a system by its supplier

This model has been integrated into a CATCH “A Compendium of Assistive Technology Checklists for the Home”. The CATCH checklist is designed to assist in the appropriate choice of assistive technology system to meet the needs of an older person in their home. CATCH provides a number of key questions which can be asked about the user’s relationship to the overall technology system as well as specific technology related (system derived) questions. The intention and purpose of CATCH is to allow social care professionals (Occupational Therapists, Social Workers, Support Workers etc) who would normally be assisting in or having input into specifying or commissioning this form of technology to determine appropriate questions. By using the CATCH checklist correctly, the user should be provided with a set of clear questions that can be used in determining the appropriateness and dependability of any particular AT system they design or are considering using.


For more information see:


Sommerville, I., Dewsbury, G. Clarke, K. and Rouncefield, M. 2002. 'A Dependability Model for Domestic Systems'. Proc. SAFECOMP 2003, Edinburgh, September 2003, Springer, 103-15.

Sommerville, I. and Dewsbury, G. 2005. Transactions paper.



Guy Dewsbury (Lancaster)


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