Experimental design: variables & controls

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The single factor you deliberately **change** (manipulate) — the cause you are testing. Plotted on the **x-axis**.

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The factor you **measure** as the outcome; it **depends on** the independent variable. Plotted on the **y-axis**.

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A factor kept **constant** in every trial so it cannot affect the result — this keeps the test **fair**.

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A **controlled variable** is a factor held **constant**. A **control treatment** is a whole **baseline run** with the tested factor **absent**, used for comparison.

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If two things change together you get a **confounding variable** — you can't tell which factor caused the result, so the test is **invalid**.

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$\%\ \text{change} = \dfrac{\text{final} - \text{initial}}{\text{initial}} \times 100$. Use the **control** value as the 'initial'.

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**Specifically**, with its quantity (e.g. 'temperature at 22 °C', 'volume of solution'), never 'keep everything the same'.