Research in a changing world – a natural experimental study
‘Natural experiment’ is a term given by public health researchers to an opportunity to observe the effects of changes in the environment, policy or practice.
Studying these changes to improve understanding of public health has a long history in the UK, one of the most famous being the physician John Snow’s study of the 1854 Broad Street cholera outbreak in London. This tradition is continued today at the CIPH by CEDAR, the Centre for Diet and Activity Research.
One focus of CEDAR’s work is to make better use of natural experiments in terms of evaluation and learning to inform future public health interventions. Altering transport infrastructure to support active travel such as walking and cycling could help to increase population levels of physical activity. A flagship grant in this area for CEDAR is the Commuting and Health in Cambridge study funded by the National Institute for Health Research (NIHR) Public Health Research programme. A key focus of this cohort study of travel behaviour and physical activity has been the launch of the Cambridgeshire Guided Busway. Opened in August 2011 and connecting St Ives andCambridge, this is the longest guided busway in the world. The study aims to assess whether providing new transport infrastructure has any effect on travel behaviour and physical activity in the commuting population – an example of a natural experimental study.
Dr David Ogilvie, principal investigator on the study says “Whilst the primary motivation for building the guided busway was to reduce congestion rather than to improve public health, we know that using public transport tends to involve at least some physical activity. In the first month alone, more than 220,000 trips were made on the busway, so the potential for a population - level effect on physical activity is worth investigating.”
Although the Busway itself was delayed, data collection for the study has been ongoing since 2009 and has provided a wealth of information about the travel and physical activity behaviours of the local commuting population. The phase two dataset is now complete and comprises follow-up survey data from some 800 participants, combined with in-depth measures of household travel in around 400 participants and objectively assessed travel and physical activity in around 200 participants. The study also includes a substantive qualitative research strand. Interviews have been completed with around 70 participants, a participant observation study has recently begun, and several mixed-method analytical projects are in progress.
Dr Ogilvie also contributed to the 2011 MRC guidance Using natural experiments to evaluate population health interventions. The aim of this guidance is to help producers, users, funders and publishers of evidence understand how and when natural experiments can be used to good effect.