Data analysis
As the study was retrospective (the sample size was fixed a
priori ) and exploratory, the 10 cases per covariate approach to sample
size calculation was used to establish, prospectively, that our sample
size of 300 was likely to be sufficient.16 Descriptive
statistics included medians and inter-quartile ranges for continuous
variables (including time-to-event variables) and counts and percentages
for categorical variables. Initial summary statistics of the whole
cohort and comparison tests between two groups, those successfully
extubated by 14 days after starting dexamethasone and those not
successfully extubated in that time, were done using GraphPad Prism 8
(GraphPad Software LLC, San Diego, CA, USA).
As the primary outcome in the present study is a binary variable, binary
logistic regression was employed. Univariate and multivariable binary
logistic regression models were fit to the data to produce unadjusted
and adjusted odds ratios, respectively, along with their corresponding
95% confidence intervals. The multivariable model included all
potentially important predictors (independent risk factors and
confounders) including clinically important variables and those with a
p<0.2 in the univariate analysis. All modelling was conducted
using the R statistical package (v4.0.2; The R Project for Statistical
Computing, www.r-project.org/) and a p-value less than 0.05 was used to
represent statistical significance throughout all inferential analyses.