Question

How does a controlled experiment work?

Answer 1

A controlled experiment is when you vary only one factor at a time. "It involves running two parallel experiments, one with the factor you are testing and one without. The experiment without the test factor is the control. For example, you want to know if bean plants grow better or worse when aspirin is added to the soil. You grow some bean plants and add aspirin to the soil, and the plants die - or they grow exceptionally well. What have you proven? Nothing! Because there are many reasons why plants might die or grow quickly, that have nothing to do with aspirin. What you should have done is grow maybe half a dozen bean plants with the aspirin and half a dozen without the aspirin, keeping everything else identical - same soil, same amount of light, same amount of water, same temperature, etc. Now, if the test group die and the control group don't - or if the test plants grown larger than the control group - you have actually demonstrated something."