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Volume 12, Number 5—May 2006


Costs of Surgical Site Infections That Appear after Hospital Discharge

Nicholas Graves*†Comments to Author , Kate Halton*†, Merrilyn Curtis*, Shane Doidge*, David Lairson‡, Marylou McLaws§, and Michael Whitby*
Author affiliations: *The Centre for Healthcare Related Infection Surveillance and Prevention, Brisbane, Queensland, Australia; †Queensland University of Technology, Brisbane, Queensland, Australia; ‡University of Texas Health Science Center at Houston, Houston, Texas, USA; §University of New South Wales, Sydney, New South Wales, Australia

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Table A3

Description of Statistical Analyses

All analyses were carried out using Stata software (Stata Statistical Software: Release 9.1, Stata Corp., College Station, TX, USA).
Evidence of multicollinearity was assessed by nested auxiliary regression, each variable was dropped from the model and the R-squared values compared to a complete model (i.e., the vif command in Stata was used to estimate variance inflation factors and the tolerances).
Nonnormality in dependent variables was assessed by visual inspection of data plots, and the Breusch-Pagan test was used as a formal test for heteroskedastic errors (i.e., the hettest command in Stata).
Terms that described interactions between socioeconomic and clinical variables were included, and the constrained model was compared with a number of unconstrained models; incremental F-tests were conducted (i.e., the test command in Stata was used).
Because the objective of the analyses was to estimate the independent effect of surgical site infections on cost outcomes, we included all available explanatory variables in a general model and did not seeks a general-to-simple reduction based on lack of statistical significance.

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