Document Detail

Sample size for regression analyses of theory of planned behaviour studies: case of prescribing in general practice.
MedLine Citation:
PMID:  17032485     Owner:  NLM     Status:  MEDLINE    
OBJECTIVES: Interest has been growing in the use of the theory of planned behaviour (TBP) in health services research. The sample sizes range from less than 50 to more than 750 in published TPB studies without sample size calculations. We estimate the sample size for a multi-stage random survey of prescribing intention and actual prescribing for asthma in British general practice. To our knowledge, this is the first systematic attempt to determine sample size for a TPB survey. METHODS: We use two different approaches: reported values of regression models' goodness-of-fit (the lambda method) and zero-order correlations (the variance inflation factor or VIF method). Intra-cluster correlation coefficient (ICC) is estimated and a socioeconomic variable is used for stratification. We perform sensitivity analysis to estimate the effects of our decisions on final sample size. RESULTS: The VIF method is more sensitive to the requirements of a TPB study. Given a correlation of .25 between intention and behaviour, and of .4 between intention and perceived behavioural control, the proposed sample size is 148. We estimate the ICC for asthma prescribing to be around 0.07. If 10 general practitioners were sampled per cluster, the sample size would be 242. CONCLUSIONS: It is feasible to perform sophisticated sample size calculations for a TPB study. The VIF is the appropriate method. Our approach can be used with adjustments in other settings and for other regression models.
Arash Rashidian; Jeremy Miles; Daphne Russell; Ian Russell
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  British journal of health psychology     Volume:  11     ISSN:  1359-107X     ISO Abbreviation:  Br J Health Psychol     Publication Date:  2006 Nov 
Date Detail:
Created Date:  2006-10-11     Completed Date:  2007-02-06     Revised Date:  2008-11-21    
Medline Journal Info:
Nlm Unique ID:  9605409     Medline TA:  Br J Health Psychol     Country:  England    
Other Details:
Languages:  eng     Pagination:  581-93     Citation Subset:  IM    
Health Services Research Unit, Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, UK.
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MeSH Terms
Asthma / drug therapy*
Drug Prescriptions*
Family Practice / methods*
Psychological Theory*
Psychology / methods,  statistics & numerical data
Regression Analysis
Social Behavior*

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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