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 examples being the physician John Snow’s study of the 1854 Broad Street cholera outbreak in London. This tradition is continued today by CEDAR, which is seeking to make better use of natural experiments in terms of evaluation and learning to inform future public health interventions.

Can changing infrastructure change behaviour?

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 and led within CEDAR by the MRC Epidemiology Unit.

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 and Cambridge, 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.

The Cambridgeshire Guided busway. Photo - Sean Hickin

The Cambridgeshire Guided busway. Photo: Sean Hickin.

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 two months, some 430,000 trips were made on the busway, so the potential for a population-level effect on physical activity is well worth investigating.”

In-depth data

Data collection for the study has been ongoing since 2009 and provides a wealth of information about the travel and physical activity behaviours of the local commuting population. Three rounds of data collection are now complete, comprising a total of over 2500 survey responses combined with over 1000 weeks of objective physical activity measurement and over 800 weeks of in-depth household travel diary records. 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. The first results from this study were published in November 2011. (Panter J, Griffin S, Jones AP, Mackett R, Ogilvie D. Correlates of time spent walking and cycling to and from work: baseline results from the Commuting and Health in Cambridge study IJBNPA 2011, 8:124.)

“There are natural experiments going on all around us,” says Dr Ogilvie. “If we can improve the way we gather evidence about their effects, the potential impact on policy and practice could be significant.”

Further information about the study can be found at