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🔬 Science · Scientific Reasoning · Lesson 2 of 6 · 5 min

Variables, controls and hypotheses: designing a fair test

⚖️ The most common reasoning question asks you to name the variable. Get the three kinds of variables straight and you will score easy points.

💡 Key idea

Change one thing (independent), measure one thing (dependent), keep everything else the same (controlled).

🧠 The idea

In a fair test, the scientist changes ONE thing on purpose: the INDEPENDENT VARIABLE (the cause, what is manipulated). The DEPENDENT VARIABLE is what is measured to see the effect; it "depends" on the independent variable. Everything else that could affect the result is held the same: these are CONTROLLED VARIABLES (constants). A CONTROL GROUP receives no treatment or the normal condition, so the results of the experimental group have something to be compared with. If two things change at once, you cannot tell which one caused the result, so the test is not valid. Results are more trustworthy with a larger SAMPLE SIZE and REPEATED TRIALS, because chance and individual differences average out. On a graph, the independent variable goes on the x-axis and the dependent variable on the y-axis.

✏️ Worked example

A student wants to know whether the amount of fertilizer changes tomato plant height. She gives four groups of 10 plants 0 g, 5 g, 10 g and 15 g of fertilizer each week. Independent variable: grams of fertilizer. Dependent variable: plant height in centimeters after six weeks. Controlled variables: plant variety, pot size, soil, water, light and temperature. Control group: the 0 g plants. Using 10 plants per group instead of 1 means one unusually tall plant cannot decide the result.

📌 Takeaways

  • Independent = what you change; dependent = what you measure.
  • Controlled variables stay the same; the control group gets no treatment.
  • More subjects and repeated trials make results more reliable.

📖 Terms in this lesson

Hypothesis: A testable prediction about how variables are related; for example, "If plants get more light, then they will grow taller."

Independent variable: The factor the researcher changes on purpose to test its effect; for example, the amount of light given to each plant.

Dependent variable: The factor that is measured to see whether it responds to the change; for example, the height each plant grows.

Control group: The group that does not receive the tested treatment, used for comparison; for example, plants kept under normal light.

✅ Check yourself

Question 1 of 3

A researcher tests whether the volume of music affects how many words people can memorize. What is the dependent variable?

  1. The type of music played
  2. The length of the test
  3. The volume of the music
  4. The number of words memorized
Show the answer and rationale

Answer: D

The dependent variable is the outcome being measured, here the number of words memorized. The volume is the independent variable the researcher changes, and the type of music and length of the test should be kept the same as controlled variables.

Question 2 of 3

Why does an experiment include a control group?

  1. To make the experiment finish faster
  2. To increase the number of variables tested
  3. To give a baseline for comparing results
  4. To guarantee the hypothesis is correct
Show the answer and rationale

Answer: C

The control group shows what happens without the treatment, so any difference in the experimental group can be linked to the independent variable. A control group does not add variables, speed the work or guarantee the hypothesis is right.

Question 3 of 3

A student tests a new plant food by giving it to one plant on a sunny windowsill and giving plain water to another plant in a dark closet. What is the main flaw?

  1. The experiment had a hypothesis.
  2. Two variables were changed at once.
  3. The plants were not measured in metric units.
  4. The test used a control plant.
Show the answer and rationale

Answer: B

Both the plant food and the amount of light differ, so any difference in growth cannot be pinned on the plant food. Using a control plant and having a hypothesis are strengths, and the units are not the main problem.

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