Evidence-appraisal glossary
Time-Varying Confounding
Confounding that changes during follow-up, where a variable is both affected by past treatment and influences future treatment and the outcome. Standard adjustment fails here because the confounder is also a step on the causal path.
Also called: treatment-confounder feedback, time-dependent confounding.
Time-varying confounding arises with treatments that change over time, such as adjusting a medication dose in response to evolving lab values that themselves respond to prior doses. Adjust for the lab value the usual way and you block part of the treatment's own effect and can introduce collider bias; ignore it and confounding remains. Special approaches known as g-methods, including marginal structural models, were designed to handle this dual role correctly.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.