📝 Control group vs experimental group (7 MCQs)
📖 From Campbell Biology • 1. Evolution and the theme of Biology and Scientific Inquiry • 7 questions available
What is Control group vs experimental group?
Definition:
A control group is a group in an experiment that is not exposed to the treatment or independent variable and serves as a baseline for comparison, while the experimental group is exposed to the treatment, and this comparison allows researchers to isolate the effect of the independent variable by ruling out other factors, ensuring that any observed differences are due to the treatment itself.
Working:
In a controlled experiment, the experimental group receives the treatment (e.g., fertilizer, drug, or light), while the control group receives a placebo or standard conditions, and all other variables (e.g., temperature, water, genetics) are kept constant between the groups; by comparing the results (dependent variable) between the two groups, researchers can determine if the treatment has a significant effect, and the use of a control group is essential for establishing cause-and-effect relationships.
Example:
A simple example is testing a new fertilizer on plant growth: the experimental group receives the fertilizer, while the control group receives no fertilizer (or a standard fertilizer), and both are grown under identical conditions; after several weeks, the height of plants is measured, and if the experimental group is taller, the fertilizer likely caused the difference, because the control group shows what happens without the fertilizer.
Reason:
Control groups are fundamental to experimental design because they provide a benchmark for interpreting results, reducing the risk of confounding variables, and ensuring that conclusions about cause and effect are valid, making them essential for rigorous scientific research.
📝 All Control group vs experimental group MCQs
Q1. A researcher tests whether a new food additive increases mouse activity. Which design best isolates the additive's effect?
📖 Explanation: The control group should experience the same conditions as the experimental group except for the factor being tested. Providing identical food without the additive allows differences in activity to be more confidently associated with the additive.
Q2. Two groups of plants receive identical light, soil, water, and temperature. Group A receives fertilizer X, while Group B receives no fertilizer. Plants in A grow taller. What is the strongest interpretation?
📖 Explanation: Because the groups were kept under comparable conditions and differed primarily in fertilizer exposure, the growth difference supports an effect of fertilizer X. However, controlled evidence supports causation without necessarily proving it with absolute certainty.
Q3. Students test whether background color affects how quickly insects are detected by predators. They place identical insects on dark and light surfaces, but the dark-surface insects are tested at night while the others are tested during daylight. What should they change?
📖 Explanation: Lighting is an additional variable that could affect predator detection, so it confounds the comparison. Keeping lighting consistent makes background color the more clearly isolated independent variable.
Q4. A researcher concludes that a treatment is effective because treated animals survived at a rate of 80%, while untreated animals survived at 70%. However, the treated animals were younger on average. What is the major flaw?
📖 Explanation: The younger age of the treated animals introduces a potential confounding variable because age may independently influence survival. Without controlling or accounting for age, the observed difference cannot be attributed confidently to the treatment.
Q5. A study records the following survival rates after exposure to a condition: Control group: 90%, Experimental group: 60%. In a repeated trial, the rates are Control: 88% and Experimental: 61%. Which conclusion is best supported by the pattern?
📖 Explanation: The experimental group has lower survival in both trials, while the control group remains higher. Repetition strengthens the evidence for an association between the treatment and reduced survival, although additional analysis would still be appropriate.
Q6. A scientist wants to determine whether a chemical changes animal behavior. Group 1 receives the chemical, Group 2 receives water, and both groups are observed for one hour. Which additional design choice would most strengthen the comparison?
📖 Explanation: Random assignment helps distribute preexisting differences between groups, while consistent observation conditions reduce alternative explanations. Together, these choices make behavioral differences more plausibly attributable to the chemical rather than selection or measurement bias.
Q7. A researcher measures average movement before and after treatment. Control movement changes from 50 to 52 units, while experimental movement changes from 50 to 80 units. The researcher claims the treatment caused a 30-unit increase. Which reasoning is most appropriate?
📖 Explanation: The control group itself increased by 2 units, suggesting that factors unrelated to treatment influenced movement. Comparing changes gives for the experimental group and for the control, producing an estimated treatment-associated difference of units.