Here is a good example of having to control for variables, in an attempt to best isolate the 2 factors being compared.
They controlled for other factors that might come into play, such as demographics and financial characteristics.
With everything else held constant, the researchers found that impatient individuals are more likely to be obese than people who are good at waiting. “We controlled for basically every variable in the kitchen sink,” says Courtemanche, the lead author and a professor at the University of Louisville. “It seems if you genuinely hold all else constant, the more patient you are, the less you weigh.”