An example of a circumstance in which correction is desired is prediction of change. Suppose the change in "x" is known under some new circumstance: to estimate the likely change in an outcome variable "y", the slope of the regression of "y" on "x" is needed, not "y" on "w". This arises in epidemiology. To continue the example in which "x" denotes blood pressure, perhaps a large clinical trial has provided an estimate of the change in blood pressure under a new treatment; then the possible effect on "y", under the new treatment, should be estimated from the slope in the regression of "y" on "x".