Answer: An independent variable, sometimes called an experimental or predictor variable, is a variable that is being manipulated in an experiment in order to observe the effect on a dependent variable, sometimes called an outcome variable.
Explanation: Example: What is a good outcome variable for deciding whether cancer treatment in a country has been improving?
A first thought might be "number of deaths in the country from cancer in one year." But number of deaths might increase simply because the population is increasing. Or it might go down if cancer incidence is decreasing. "Percent of the population that dies of cancer in one year" would take care of the first problem, but not the second.
This example makes the point that a rate is often a better measure than a count.
