Before revision:  If measurements of input X and output Y are confounded, the causal relationship can nonetheless be identified if an instrumental variable Z is available that influences X directly, but is conditionally independent of Y given X  . The classic two-stage least squares algorithm (2SLS) simplifies the estimation problem by modeling all relationships as linear functions.
After revision:  If measurements of input X and output Y are confounded, the causal relationship can nonetheless be identified if an instrumental variable Z is available that influences X directly, but is conditionally independent of Y given X and the unmeasured confounder . The classic two-stage least squares algorithm (2SLS) simplifies the estimation problem by modeling all relationships as linear functions.