The Fertilizer Fair Test
Ms. Ortiz's fifth-grade class is testing whether fertilizer helps bean plants grow. Group A puts four bean plants in identical cups on the windowsill. Each plant gets 50 mL of water mixed with fertilizer every morning. Group B puts four bean plants in identical cups on a shelf across the room, away from the window. Each plant gets 50 mL of plain water every morning. After three weeks, Group A's plants average 18 cm tall, and Group B's average 11 cm. A student concludes, "Fertilizer made the plants grow taller, so we proved it works." Another student points out that the two groups were not treated the same way. The class has one more month and enough supplies to run one more test.
1. Identify the flaw in the experiment and explain how it affects the conclusion.
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The experiment changes two things at once: whether the plants get fertilizer and where the plants sit (windowsill versus dark shelf). Group A got both fertilizer and more light. Since light also affects plant growth, the 7 cm difference could be caused by light, fertilizer, or both. A fair test changes only one variable, the fertilizer, while keeping light, water amount, cup size, and soil the same. Because the variables are mixed together, the class cannot conclude that fertilizer caused the extra growth, even though the fertilizer group did grow taller.
2. Design a better test using the same supplies, and describe the data you would collect and how you would decide whether fertilizer helps.
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Put all eight plants on the same windowsill so every plant gets the same light. Give four plants 50 mL of plain water and four plants 50 mL of fertilizer solution, using the same cups and soil. Rotate the cups every few days so no plant is always in the same spot. Measure each plant's height in centimeters every week and record it in a table. Also note leaf color and number of leaves. After several weeks, compare the average heights. If the fertilized plants are consistently taller under the same light and water conditions, that is evidence fertilizer helps. If the averages are about the same, the original result was probably caused by light.
3. One plant in Group A wilts and dies. A student wants to remove its height from the data because it makes Group A's average lower. What should the class do, and why?
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The class should not simply delete the plant because its result is inconvenient. That would bias the data toward the fertilizer claim. They should first look for a cause: was the cup knocked over, was it not watered, or did it get a disease? If the death came from an accident unrelated to the fertilizer, they can report the problem and explain why they excluded that plant. If the cause is unknown, they should keep the measurement, report the death, and calculate the average both with and without it. Good scientists report problems and let others see the evidence rather than hiding data that does not fit.