The Tavistock Institute, in partnership with AECOM, were commissioned by the Department for Transport to develop a set of guidance for those commissioning and undertaking impact evaluations in the transport sector.

This addresses one of the main challenges in undertaking an evaluation: how to demonstrate that the observed outcomes and impacts have been caused by the intervention, confidently ruling out the influence of external factors.

The guidance provides a step by step guide for considering the most suitable evaluation approach to take, taking into account the type of intervention being delivered, the wider context within which it is taking place and the evidence requirements. It is not intended to be prescriptive but provide a flexible framework for selecting the best overarching evaluation approach.

Although primarily designed for the transport sector, those commissioning and undertaking evaluation in other sectors may also find this guidance useful. Although not covering all aspects of evaluation, it does provide an in depth comparison of the strengths and weaknesses of different evaluation designs and the circumstances under which each might be chosen, focusing particularly on simple outcome, experimental and theory based approaches.

The earlier steps in the guidance – covering the value of undertaking a ‘mapping’ of the underlying logic of a project or programme – can be applied to all types of evaluation.