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📝 Independent vs dependent variables in biology experiments (7 MCQs)

📖 From Campbell Biology • 1. Evolution and the theme of Biology and Scientific Inquiry • 7 questions available

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What is Independent vs dependent variables in biology experiments?

Definition:
In biology experiments, the independent variable is the factor that the researcher manipulates or changes to observe its effect, while the dependent variable is the factor that is measured or observed to determine the effect of the independent variable; the dependent variable depends on the independent variable, and this relationship is central to testing hypotheses and establishing cause-and-effect relationships.

Working:
The independent variable is set by the experimenter (e.g., drug dosage, temperature, light intensity), and the dependent variable is the outcome (e.g., enzyme activity, growth rate, number of offspring); the relationship is often expressed as $Y = f(X)$ , where $X$ is the independent variable and $Y$ is the dependent variable; controlled variables (held constant) ensure that only the independent variable affects the dependent variable, and the data are analyzed to see if changes in $X$ correlate with changes in $Y$ , with the null hypothesis being no effect.

Example:
A simple example is an experiment testing the effect of salt concentration on the growth of bacteria: the salt concentration (e.g., 0%, 1%, 5%) is the independent variable, and the number of bacterial colonies after 24 hours is the dependent variable; the relationship is analyzed, and if bacterial growth decreases with increasing salt, the independent variable affects the dependent variable, demonstrating the cause-and-effect relationship.

Reason:
Understanding independent and dependent variables is fundamental for designing experiments, interpreting results, and understanding biological cause-and-effect, and it is a core concept in scientific methodology that underpins all experimental research.

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📝 All Independent vs dependent variables in biology experiments MCQs

Q1. A student investigates whether different amounts of fertilizer affect the height of bean plants. All plants receive the same water, soil, light, and pot size. Which pairing correctly identifies the independent and dependent variables?

A.Fertilizer amount is independent; plant height is dependent ✅

B.Plant height is independent; fertilizer amount is dependent

C.Water amount is independent; fertilizer amount is dependent

D.Pot size is independent; plant height is dependent

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The independent variable is the factor deliberately changed by the researcher, which is fertilizer amount. The dependent variable is the measured response, plant height. Keeping water, soil, light, and pot size constant helps isolate the fertilizer effect.

Q2. A researcher tests whether temperature influences enzyme activity. Samples are placed at $10^\circ C$ , $20^\circ C$ , $30^\circ C$ , and $40^\circ C$ , while enzyme concentration and substrate concentration remain constant. Why is enzyme activity considered the dependent variable?

A.It is the factor deliberately manipulated

B.It is measured to determine the response to temperature ✅

C.It must remain constant during the experiment

D.It determines which temperatures are selected

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Temperature is deliberately manipulated, so it is the independent variable. Enzyme activity is measured after exposure to each temperature and therefore represents the response. A dependent variable changes or is expected to change as the independent variable changes.

Q3. A farmer compares three irrigation schedules and records crop yield after harvest. However, the fields also differ substantially in soil fertility. The farmer concludes that irrigation schedule alone caused the yield differences. What is the strongest criticism?

A.Crop yield cannot be used as a dependent variable

B.Irrigation schedule should always be the dependent variable

C.Soil fertility is an uncontrolled variable that could influence crop yield ✅

D.The experiment has no independent variable

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Irrigation schedule is the intended independent variable and crop yield is the dependent variable, but soil fertility is a confounding factor. Because fertility differs between fields, yield differences cannot confidently be attributed solely to irrigation.

Q4. A graph shows that as study time increases from $1$ to $5$ hours, average test score rises from $55$ to $82$ . A student says, 'Because test score is on the vertical axis, it must be the independent variable.' Which response is most scientifically appropriate?

A.The student is correct because vertical-axis variables are always independent

B.The student is incorrect because the variable deliberately varied is independent, regardless of graph position ✅

C.The student is correct because scores determine study time

D.Both variables are automatically dependent when shown on a graph

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Axis position alone does not determine variable status. In this investigation, study time is the factor varied or compared, making it independent, while test score is the measured outcome. Graph conventions commonly place the independent variable horizontally, but the experimental design is decisive.

Q5. Two groups of plants receive identical light and water. Group A receives $2$ g of fertilizer and Group B receives $6$ g. Their average heights are measured weekly. Which experimental design change would most directly improve the conclusion about fertilizer amount affecting height?

A.Change both fertilizer amount and watering schedule between groups

B.Measure plant height before and after treatment and use several plants per treatment ✅

C.Use only the tallest plant from each group for comparison

D.Allow each group to receive different amounts of sunlight

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Measuring initial and later heights helps account for starting differences, while using several plants reduces the influence of individual variation. Changing additional conditions would introduce confounding variables and weaken the causal interpretation.

Q6. Suppose a graph of medication dose $x$ versus average recovery time $y$ shows recovery time decreasing as dose increases, but after a certain dose the curve becomes nearly horizontal. Which interpretation best respects the variables and the pattern?

A.Dose is dependent because recovery time changes with it

B.Recovery time is independent because it appears on the vertical axis

C.Dose is independent and recovery time is dependent; additional dose produces little further reduction after the plateau ✅

D.Neither variable can be identified because the graph contains a curved relationship

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The dose is the manipulated or explanatory variable, whereas recovery time is the measured outcome. The flattening curve suggests diminishing response at higher doses, meaning additional increases in dose produce progressively smaller changes in recovery time.

Q7. A scientist claims that increasing exercise duration causes lower resting heart rate because people who exercise longer generally have lower heart rates. Which additional test would most strongly distinguish a causal relationship from a simple association?

A.Ask participants whether they personally believe exercise is beneficial

B.Randomly assign participants to different exercise durations while controlling other major conditions ✅

C.Measure only participants who already exercise regularly

D.Compare heart rates without recording exercise duration

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Random assignment to different exercise durations makes the exercise condition the independent variable while resting heart rate becomes the dependent variable. Controlling other important conditions reduces alternative explanations, providing stronger evidence for a causal relationship.

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