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Open Access Highly Accessed Open Badges Research article

Process evaluation for complex interventions in primary care: understanding trials using the normalization process model

Carl R May1*, Frances S Mair2, Christopher F Dowrick3 and Tracy L Finch1

Author Affiliations

1 Institute of Health and Society, Newcastle University, 21 Claremont Place, Newcastle upon Tyne, NE2 4AA, UK

2 Division of General Practice and Primary Care, University of Glasgow, Glasgow, UK

3 School of Population, Community and Behavioural Sciences, University of Liverpool, Liverpool UK

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BMC Family Practice 2007, 8:42  doi:10.1186/1471-2296-8-42

Published: 24 July 2007



The Normalization Process Model is a conceptual tool intended to assist in understanding the factors that affect implementation processes in clinical trials and other evaluations of complex interventions. It focuses on the ways that the implementation of complex interventions is shaped by problems of workability and integration.


In this paper the model is applied to two different complex trials: (i) the delivery of problem solving therapies for psychosocial distress, and (ii) the delivery of nurse-led clinics for heart failure treatment in primary care.


Application of the model shows how process evaluations need to focus on more than the immediate contexts in which trial outcomes are generated. Problems relating to intervention workability and integration also need to be understood. The model may be used effectively to explain the implementation process in trials of complex interventions.


The model invites evaluators to attend equally to considering how a complex intervention interacts with existing patterns of service organization, professional practice, and professional-patient interaction. The justification for this may be found in the abundance of reports of clinical effectiveness for interventions that have little hope of being implemented in real healthcare settings.