Table 1 Description of ‘dimensions of complexity’ in the proposed intervention study as per Medical Research Council Guidance [1]

Number of and interactions between components within the experimental interventions

Since we anticipate that no two individuals will have the same impairments, the assessment and management of impairment versus functional need will be undertaken by different individuals due to differing skill sets. For example, vision and hearing impairment requiring devices (glasses and hearing aids) will be undertaken by specialist clinicians (optometrists, ophthalmologists and audiologists), whereas the functional ability and environmental context, caregiver assessment and the resulting management strategy will be undertaken by a sensory support worker

Number and difficulty of behaviours required by those delivering or receiving the intervention

Whereas the clinicians will assess and correct hearing and vision impairments according to their standard good practice guidelines, the sensory support worker will undertake newly learned protocols, and will have to choose from a variety of elements, likely within different modules including: vision training, auditory training, caregiver education and training, information delivery, sign-posting and environmental assessment and correction

Number and variability of outcomes

In order to fulfil the remit of improving ‘mental well-being’ in elderly EU citizens, various outcomes need to be captured, including health-related quality of life (QoL), improved functional ability, social connectivity, caregiver factors, attainment of personal goals, as well as more easily quantifiable factors such as cognitive performance, level of depression and other behavioural disturbances.

Degree of flexibility or tailoring of the intervention permitted Implications for development and evaluation

Since no two individuals will have the same degree of cognitive and sensory impairment and functional ability, the intervention will have to be highly tailored and individualised, albeit within the structure of a reproducible, manualised and modular approach in which each person with dementia: caregiver dyad will be offered each module of the intervention package

A good theoretical understanding is needed of how the intervention causes change, so that weak links in the causal chain can be identified and strengthened

The background literature of existing evidence of potential mechanisms, as well as clinical experience, has suggested that each aspect of the intervention can be linked to identifiable intermediate impacts and final outcomes, and can be outlined in a logic model [24]