Numerous observational studies have attempted to identify risk factors for infection with SARS-CoV-2 and COVID-19 disease outcomes. Studies have used datasets sampled from patients admitted to hospital, people tested for active infection, or people who volunteered to participate. Here, we highlight the challenge of interpreting observational evidence from such non-representative samples. Collider bias can induce associations between two or more variables which affect the likelihood of an individual being sampled, distorting associations between these variables in the sample. Analysing UK Biobank data, compared to the wider cohort the participants tested for COVID-19 were highly selected for a range of genetic, behavioural, cardiovascular, demographic, and anthropometric traits. We discuss the mechanisms inducing these problems, and approaches that could help mitigate them. While collider bias should be explored in existing studies, the optimal way to mitigate the problem is to use appropriate sampling strategies at the study design stage.

Acknowledgements: We would like to thank the editor, Mike Pratt, and the three anonymous reviewers for their insightful comments. We would also like to acknowledge the help we received from Paula Jarzabkowski and members of OTREG on earlier drafts, and the support of the Novak Druce Center for Professional Service Firms at the University of Oxford. 2 FROM PRACTICE TO FIELD: A MULTI-LEVEL MODEL OF PRACTICE-DRIVEN INSTITUTIONAL CHANGE ABSTRACTThis paper develops a model of practice-driven institutional change; that is, change that originates in the everyday work of individuals, but results in a shift in field-level logic. In demonstrating how improvisations at work can generate institutional change, we attend to the earliest moments of change that extant research neglects; and we contrast existing accounts that focus on active entrepreneurship and the contested nature of change. We outline the specific mechanisms by which change emerges from everyday work, becomes justified, and diffuses within the organization and field, as well as precipitating and enabling dynamics that trigger and condition these mechanisms.3