These last two observations confirmed that there were very few if no confounders like PCR errors, chimeras and pseudogenes involved in this genetic diversity.
If there are no confounders in the environment unique to each twin, association within MZ twin pairs are estimates of causal effects [ 48, 49].
If a confounder was included as a P-spline, we checked whether a polynomial led to a smaller AIC.
If a confounder, the observed association between interpersonal discrimination and mental health will be biased or altogether spurious.
If a confounder is taken into account but is dichotomised, this may remove some but not all of the effects of confounding and hence still result in bias.
If a confounder changed the estimates of the total microbial quantity score by more than 10%, all the models of the outcome were adjusted for the factor.
This adjustment was made available for sensitivity analysis; specifically, to explore how much the sample size result could change if the confounder were in fact an effect modifier.
Adjustment then is used to give the likelihood of adverse outcome if the confounder were imagined to be equally distributed between groups.
For example, if a confounder causes a second variable with a high probability (i.e. the second variable is a strong marker for the confounder), including the marker for the confounder should reduce bias [ 6].
Although, intuitively, the "best" adjustment set is expected to be {C1,C2*,C3*}, adjusting on a mismeasured confounder may increase bias under certain conditions [ 42, 43] such as the presence of a qualitative interaction between exposure and confounder if the confounder is binary [ 43].
