Probably interesting is the research of Colley, Hodkinson et al (see below). They show very confincingly that every learning and teaching activity has formal as well as informal aspects. Therefore they don’t speak about formal and informal learning, but about the formality of learning activities. Based on a set of characteristics of learning, one can ‘measure’ the formality. Really interesting research and for me – being a consultant – and my customers very helpful.

Colley, H., P. Hodkinson, et al. (2003). Informality and formality in learning: a report for the Learning and Skills Research Centre. Leeds: Life Long Learning Institue, University of Leeds.