Which Bean Team Should the Class Trust?
Two fifth-grade teams want to find out whether liquid plant food makes bean plants grow taller. Team A plants 10 bean seeds in one flat tray on the windowsill, waters every plant with 50 mL of water mixed with plant food every other day, and measures heights after three weeks. Team B plants 10 seeds in two trays. Five seeds go in a tray on the windowsill and get 50 mL of plain water every other day. Five seeds go in a tray on a shelf across the room and get 50 mL of water with plant food every other day. Team B reports that the shelf plants grew 4 cm taller on average. The class argues about whose results to trust.
1. Which team's plan is closer to a fair test, and what is still wrong with it?
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Team B is closer, because it has something to compare against: a group that got plain water. Team A gave plant food to every plant, so even if all 10 plants grew tall, there is nothing to show what would have happened without plant food. But Team B has a serious flaw: the plain-water tray sat on the windowsill and the plant-food tray sat on a shelf across the room. Light and warmth were different for the two groups, so the 4 cm difference could come from light rather than from plant food. A fair test changes only one thing (plant food) and keeps everything else the same - same light, same temperature, same soil, same amount of water.
2. A student says Team B's 4 cm difference 'proves plant food works.' Give two other explanations for the 4 cm that the evidence cannot rule out.
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- Light. The windowsill tray may have gotten less direct sun or more shade than the shelf, and plants grow differently in different light. That alone could account for 4 cm.
- Temperature or drafts. A shelf across the room may be warmer or cooler, or near a door or vent, which changes how fast plants grow and how much water they lose. Neither explanation has been tested, so the 4 cm cannot be credited to plant food. The evidence is consistent with several causes, and the experiment cannot separate them.
3. Describe the change you would make to Team B's setup and explain what result would then support the claim that plant food helps.
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Put both trays in the same place - for example, side by side on the same windowsill - so both groups get the same light and temperature. Keep water amount, soil, pot type, seed type, and watering schedule identical, and change only whether plant food is added. Use more than 5 plants per group so one oddly small seed does not swing the average. If the plant-food group then grows taller on average than the plain-water group, and the same thing happens when the whole setup is repeated, the class has good evidence that plant food made the difference.