A number of texts describe the requirements for single-case evaluation, and emphasise various aspects according to each other’s perspective. A hallmark of this methodology is that the minimum requirements are very basic indeed, and at the same time could be made as complex as one prefers. Apart from the degree of experimentation, a related means of categorising single-case designs is the more obvious use of phases, which can be described as ‘the periods of time during which distinctive evaluation activities occur’. Hence, a key question determining the classification of single-case designs is internal validity, i.e., the extent to which an ‘experiment rules out alternative explanations to results’. Threats to internal validity are ‘the factors or influences other than the intervention that could explain the results’ such as maturation and influence of history. An almost infinite number of design options are available in single-case evaluative research.