An independent variable is variable whose effect on the dependent variable (outcome) is examined in a regression analysis.

Example 1:

Obesity is a common health problem which may be measured by waist circumference. For the purpose of formulating prevention strategy, one may be interested in exploring factors associated with obesity.

Aoyagi et al. (2006) identified an association of dyslipidemia with obesity. In this case, dyslipidemia is taken as an independent variable to explain obesity.

Example 2:

It is sometimes said that the brain size may have influences on intelligence. Brain size can be measured by horizontal resonance images (MRI) while intelligence may be indicated by an Intelligence Quotient (IQ) score.

As we are interested in determining if brain size can explain the difference of intelligence among different people, MRI is our independent variable for examining its effects on the dependent variable of IQ.