The authors concluded that, despite a comprehensive confounder
adjustment, the observed difference in the mortality risk might be attributable to residual or unmeasured confounding; in particular, the lack of information on the severity of the underlying disease and associated risk factors might have influenced the decision on the PCI setting.
If you think your exposure might act differently on the outcome by sex, you should think about confounders
that might also act differently by sex, and account for that," says Jessie Buckley, an assistant professor at the Johns Hopkins Bloomberg School of Public Health, who coauthored the new study.
Figure 1d describes an extension of Figure 1c, where now the time-varying confounder
is affected by prior exposure.
Since sensation seeking is not often included in behavioural surveys of populations at risk for HIV, (5) it would be an unmeasured confounder
in many studies and could not be adjusted for using conventional statistical methods.
More specifically, the SI assumption implies the following requirements: 1) there should be no latent confounders
in the X [right arrow] Y path (all of the variables that cause both X and Y must be included in the model), 2) there should be no latent confounders
in the M [right arrow] Y path (all the variables that cause both M and Y must be included in the model), 3) there should be no latent confounders
in the X [right arrow] M path, and 4) treatment cannot be a cause of any confounder
of the M [right arrow] Y path.
Fisher was very aware that the possible presence of unknown confounders
invalidates epidemiological studies as a way of proving cause and effect relationship (6).
The investigators then pooled adjusted data from the studies accounting for the effects of various confounders
, and found a combined reduction in the risk for death after pneumonia of 34% in statin users compared with nonusers, which also was statistically significant, said Dr.
Analysis of covariates indicated that gender, religion, and residence largely had no effect on the associations between pathogens; however, in some instances, age and season were identified as confounders
or effect modifiers (Table 2).
A sensitivity analysis determines the magnitude of a potential unmeasured confounder
that could erase the observed association or would need to be present to materially alter the conclusions of a study.
The conclusion that nonsedating antihistamines and antibiotics contribute to CRS-related fatigue rests, as noted in the article, on the variable chosen as a control for the confounder
of disease symptomatic severity.
This is a potential confounder
very rarely addressed in studies of this type.
That particular confounder
was the whole reason the Ranch Hand study could even be done.