Biology · Introductory biology · Worked example
Fix a flawed experiment
A student tests whether a fertilizer makes bean plants grow taller. She gives fertilizer to one plant on a sunny windowsill and none to one plant in a shaded corner. After three weeks the fertilized plant is 4 cm taller. Name the independent and dependent variables, find two flaws, and redesign the experiment.
Name the variables
The independent variable is fertilizer, given or not. The dependent variable is plant height after three weeks.
Find the confounder
The plants also differ in light: one sits in sun and the other in shade. The 4 cm could come from the light, not the fertilizer, so light must be held the same.
Find the missing replication
With one plant per group, the difference could come from those two plants, for example from seed size or genetics, rather than from the treatment.
Redesign
Grow 20 seedlings of one variety on one bench, with the same soil, pots, water and light. Assign 10 at random to fertilizer and 10 to water only, the control. Measure every plant the same way after three weeks and compare the group means. If the fertilized mean is 18.4 cm and the control mean 15.9 cm, the difference is 2.5 cm, and a statistical test can say whether that is larger than chance.
Result
The independent variable is fertilizer and the dependent variable is plant height. The flaws: light differs between the plants, a confounder, and there is only one plant per group, so no replication. Redesign: many seedlings of one variety under the same conditions, assigned at random to fertilizer or water only, with the group means compared.
Your turn
A study finds that students who drink more coffee get lower grades. Can you conclude that coffee lowers grades?
Show the answer and explanation
No.
This is an observational study. A confounder, such as staying up late, could raise coffee drinking and lower grades. Only an experiment that assigns students to coffee or no coffee at random could test cause.
Keep exploring
The redesigned experiment opens in the Experiment planner; change a group size or add a controlled variable and see how the design summary changes.
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