Designing a Controlled Experiment
Planning fair tests with variables, controls, and evidence
Designing a Controlled Experiment
Planning fair tests with variables, controls, and evidence
Science - Grade 9-12
- 1
A student wants to test whether adding nitrogen fertilizer affects the height of bean plants after 4 weeks. The student grows 20 bean plants, gives half of them fertilizer, gives the other half no fertilizer, and keeps the same pot size, soil type, water amount, and light exposure. Identify the independent variable, dependent variable, control group, and at least two constants.
The independent variable is what the researcher changes on purpose.
The independent variable is whether the plants receive nitrogen fertilizer. The dependent variable is plant height after 4 weeks. The control group is the plants that do not receive fertilizer. Constants include pot size, soil type, water amount, and light exposure. - 2
Write a testable hypothesis for an experiment investigating how the amount of daily sunlight affects the growth rate of tomato plants.
Use an if-then-because structure that connects the independent variable to the dependent variable.
A testable hypothesis is: If tomato plants receive more hours of daily sunlight, then their growth rate will increase because sunlight provides energy for photosynthesis. - 3
A researcher tests whether caffeine affects reaction time. One group drinks coffee with caffeine, and another group drinks coffee without caffeine that tastes the same. Explain why the decaffeinated coffee group is needed.
The decaffeinated coffee group is the control group. It helps show whether any change in reaction time is caused by caffeine rather than the taste of coffee, the act of drinking coffee, or the participants' expectations. - 4
A student tests whether music helps people memorize vocabulary words. The music group studies in a quiet classroom, while the no-music group studies in a noisy hallway. Identify the problem with this design and explain how to fix it.
A controlled experiment should change only one main factor between groups.
The study has a confounding variable because the study locations are different. The student should have both groups study in the same type of environment, with music as the only planned difference. - 5
In an enzyme experiment, a student tests the effect of temperature on reaction rate using only one trial at 20 degrees Celsius, one trial at 30 degrees Celsius, and one trial at 40 degrees Celsius. Explain why this design is weak and how to improve it.
The design is weak because one trial at each temperature may be affected by random error. The student should run multiple trials at each temperature and calculate the average reaction rate for each temperature. - 6
A scientist is testing a new plant hormone on 100 seedlings. Explain why randomly assigning seedlings to the treatment group and control group improves the experiment.
Random assignment helps prevent bias when forming groups.
Random assignment helps make the groups similar before the treatment begins. It reduces the chance that differences in seedling size, health, or genetics will unfairly affect the results. - 7
A student records the following average bacterial colony counts after applying different amounts of disinfectant: 0 mL: 120 colonies, 1 mL: 75 colonies, 2 mL: 40 colonies, 3 mL: 18 colonies. Identify the independent variable, dependent variable, and the best graph type for displaying the data.
The x-axis usually shows the independent variable, and the y-axis usually shows the dependent variable.
The independent variable is the amount of disinfectant in milliliters. The dependent variable is the average number of bacterial colonies. A line graph or scatter plot would be a good choice because the amount of disinfectant is a numerical variable. - 8
A drug company tests a new allergy medicine. Some participants receive the medicine, and others receive a sugar pill that looks identical. Neither the participants nor the doctors collecting the data know who received which pill. Name and explain the two design features used here.
The sugar pill is a placebo, which helps control for the effect of expectations. The study is double-blind because neither the participants nor the doctors collecting data know who received the medicine, which reduces bias. - 9
Design a controlled experiment to test which of three paper towel brands absorbs the most water. Include the independent variable, dependent variable, control variables, and a basic procedure.
Keep the size of each towel sample the same so the comparison is fair.
The independent variable is the paper towel brand. The dependent variable is the amount of water absorbed. Control variables should include towel size, water volume, soaking time, and method of measuring absorbed water. A basic procedure is to place equal-size sheets into the same amount of water for the same time, remove them the same way, measure how much water remains or how much mass was gained, and repeat trials for each brand. - 10
A class concludes that a fertilizer makes plants grow taller because the fertilized plants averaged 16 cm tall and the unfertilized plants averaged 15.5 cm tall after 3 weeks. The experiment had only two plants in each group. Explain why the conclusion should be cautious.
The conclusion should be cautious because the difference in average height is small and the sample size is very low. With only two plants per group, random variation could explain the difference, so more plants and repeated trials are needed. - 11
A biologist observes that birds visit feeders more often on cold days than warm days. Explain why this observation alone does not prove that cold temperature causes more feeder visits, and describe a controlled experiment that could test the relationship.
Observational data can show a pattern, but a controlled experiment is stronger for testing cause and effect.
The observation does not prove causation because other factors, such as food availability, wind, or time of day, might affect feeder visits. A controlled experiment could use identical feeders in controlled environments where temperature is changed while food type, feeder design, light, and observation time are kept constant. - 12
A student wants to test whether more light increases algae growth in aquariums. List four variables that should be kept constant.
Variables that should be kept constant include water volume, starting amount of algae, nutrient level, water temperature, aquarium size, and type of light. Keeping these constant helps make light exposure the main factor being tested. - 13
A student wants to test whether sleep affects test scores by asking classmates to sleep only 3 hours before a test. Explain the ethical problem and suggest a safer alternative design.
Human studies should avoid harm and require informed consent.
The ethical problem is that intentionally restricting sleep could harm participants and affect their well-being. A safer alternative is to collect voluntary survey data about students' normal sleep from the previous night and compare it with test performance, while protecting privacy. - 14
Write a short procedure for testing how ramp angle affects the speed of a toy car. Include how you would measure the dependent variable and at least three variables you would control.
Do not push the car because that would add another variable.
A good procedure is to release the same toy car from the same starting point on a ramp set at different angles, such as 10, 20, and 30 degrees. The dependent variable is speed, which can be measured by timing how long the car takes to travel a fixed distance and calculating speed. Controlled variables should include the car, ramp surface, starting position, distance measured, and release method. - 15
An experiment comparing two water filters reports the results from one test only. Filter A removed 92 percent of particles, and Filter B removed 89 percent. Explain what additional evidence would make the claim that Filter A is better more reliable.
The claim would be more reliable if the filters were tested in many repeated trials using the same starting water quality and water volume. The results should include averages and variation, such as range or standard deviation, to show whether the difference is consistent and meaningful.