Evidence-appraisal glossary
Instrumental variable
An instrumental variable is a factor used in observational research that influences whether people get a treatment but affects the outcome only through that treatment, and shares no common cause with it. Researchers use it to estimate causal effects while reducing distortion from unmeasured confounding.
Also called: IV analysis, instrumental variables estimation, two-stage least squares.
What it is
An instrumental variable (IV) is a clever workaround for a core problem in observational studies: people who receive a treatment often differ from those who do not in ways you cannot fully measure. A valid instrument is a variable that (1) reliably shifts who gets treated, (2) affects the outcome only through the treatment, and (3) shares no common cause with the outcome. Because the instrument nudges treatment in a quasi-random way, comparisons across its levels can approximate a randomized experiment.
How to use it when reading a study
Check the three assumptions, because two of them cannot be proven from data alone. Ask: is the instrument strongly linked to treatment (a "weak instrument" gives unstable, biased estimates)? Is the claim that it touches the outcome only via treatment plausible? IV estimates apply to those whose treatment the instrument actually moved, so generalize cautiously. Mendelian randomization is one common IV design.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.