Applying a longitudinal tracer methodology to evaluate complex interventions in complex settings

Long-running multi-faceted intervention studies are particularly problematic in large and complex organizations where traditional methods prove too resource intensive and can yield inaccurate and incomplete findings. This paper describes the first use of, longitudinal tracer methodology (LTM), a realist approach to evaluation, to examine the links between multiple complex intervention activities (intervention processes) and their outcomes on a construction megaproject. LTM is especially useful when the researcher has little control over intervention delivery but has access to evidence drawn from a variety of sources to evaluate the effects of intervention activities over time. There are, however, very few examples of how this methodology can be successfully deployed in complex organisational settings and none on a construction megaproject. In this paper we present a case study of its use over a period of three years, on 24 construction sites forming London’s Thames Tideway Tunnel (Tideway) megaproject. The aim of the study was to examine the ‘transformational’ power of occupational safety and health (OSH) interventions as they played out across the multiple organisations and supply chains that constituted the megaproject. The case study shows how, with careful design, the method can be adapted in-flight to accommodate shifting lines of inquiry as the intervention activities progress and change. This feature of the method, along with its resource efficient operation, make it a particularly attractive option where interventions are likely to have differential effects across multiple sites of enactment.