Several techniques can be used to eliminate the influence of confounders
, such as matching or restriction in the design of a study.
If that assumption is true, then the instrument represents variations in local pollutants that are randomized with respect to confounders
, measured or unmeasured, and therefore provides a causal estimate of the effect of local air pollution concentrations.
In an HDPS analysis, the underlying database of the study is automatically searched for hundreds of empirical confounders
, which are used in the subsequent analyses.
(5) It also suffers from many other confounders
of distance IVs, a small list of which are reproduced in Table 1 below.
We did not have information on confounders
or PFAA concentrations for the previous pregnancy in parous women, and were only able to examine the potential impact of uncontrolled confounding in a simple sensitivity analysis that assumed that the influence of all unmeasured or unknown potential confounders
could be represented by a single dichotomous confounder
Conventional statistics are readily implemented provided that confounders
are measured once, at the start of the follow-up.
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.
Second, the contribution of all potential predictors to the association between adjustment latitude and sickness presence was assessed blockwise in the 4 confounder
groups: resources (health and finances), alternative strategies (compensatory leave/vacation and sickness absence), work (attendance requirements and job demands) and personal (loyalty).
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.
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.
Effect of covariates on gastrointestinal pathogen associations * Covariate Pathogens Age Season Gender Vibrio cholerae/rotavirus Confounder
Interaction No effect V.
The study authors asked that researchers note that this could be a confounder
when screening for prostate cancer, and advise caution in interpreting PSA results from patients who are taking any of these medications, especially long term.
An example of a sensitivity analysis (26) performed to account for potential residual confounding caused by a hypothetical unmeasured confounder
is reported in Table 5.
To illustrate assumptions underlying these approaches, we consider a scenario in which we are interested in assessing relationships between a continuously measured exposure (X), a binary confounder
(Z), a binary indicator of sex (S), and a continuous outcome of interest (Y).
A possible study confounder
is the potential for the "sick quitter" syndrome, in which patients might have quit drinking post MI because their health was deteriorating to a greater extent.