Limitations of retrospective claims analyses derive primarily from the absence of randomization, confounding outcomes with the different baseline characteristics and prognoses between cohorts and risking issues of internal and potentially external validity, 78 which limits conclusions regarding causality.
These factors support the premise that AR itself should be considered an intervention, rather than considering AR processes as confounding outcomes measurement or staining the purity of research quality.
For the ORIF procedure the different fracture patterns, as well as the use of a medial calcar butress plate, may confound outcome.
We believe that differential use of tidal volume, PEEP and recruitment manoeuvre (open lung technique) between the study and control arms may confound outcome, and so we will perform subgroup analysis on these variables.
The efficacy and safety of therapies in the "average patient" have been historically obtained using randomized clinical trials where the variable patient attributes are randomly distributed so as not to confound outcome measurements.
The present study was a retrospective analysis of data obtained from EMR; therefore, although the known variables were adjusted for in the regression analyses, unidentified factors may have confounded outcome measures.
In the quantile quantile plots, correlations approaching normality may hide substructures that confound outcome or, alternatively, deviations from normality may suggest substructure differences that may not actually exist, which can often be corrected (Voorman et al. 2011).
Channeling, selection, and other biases inherent in nonrandomized studies may confound outcomes.
Surgical co-intervention, such as a general, neurosurgical or orthopaedic procedure, were likely to confound outcomes.
