Independent, Dependent and Controlled Variables

Every passage describing an experiment is built around one thing being changed on purpose. That's the independent variable — the condition the scientist sets to different values, like light exposure running from 2 hours to 14 hours, or nitrate concentration at 10, 20, 30, and 40 mg/L. The dependent variable is the outcome read back out afterward: stem height, leaf area, reaction time. Spot the one column that was deliberately varied and the one column that was measured as a result, and you've named both.

Everything else in the setup — soil, water volume, temperature, trial length — has to be matched across every group. These are controlled variables, and pinning them down is what lets you blame the result on the independent variable alone. If two conditions differ at once, a changed outcome could be explained by either one, and the experiment can't tell you which. That's why a sentence listing matched conditions matters just as much as the one describing what changed.

This is also where experiments go wrong without anyone noticing. A variable the scientist never intended to touch can still drift during a run — a room warming up over an afternoon, for instance — and ride along with the variable actually being tested. When that happens, a measured variable and a controlled variable get confused, or an uncontrolled one sneaks in disguised as background. Comparing two experiments on this unit usually comes down to checking which one kept its controls honest.