The Watering Schedule Investigation
A fifth-grade class is testing whether the amount of water affects how tall bean plants grow. They plant 12 identical bean seeds in identical pots with the same soil and place them on the same windowsill. Every day for three weeks, Group A gives 3 plants 50 mL of water, Group B gives 3 plants 100 mL, Group C gives 3 plants 150 mL, and Group D gives 3 plants no water. All plants get the same amount of light and are kept at the same temperature. After three weeks, the students measure plant height in centimeters. Group C's plants are the tallest. One student says, "This proves 150 mL is the perfect amount of water for every plant everywhere."
1. Identify the independent variable, dependent variable, and at least two controlled variables in this investigation. Then explain whether the student's claim that 150 mL is "perfect for every plant everywhere" is justified by this evidence.
Read the worked answer
Independent variable: the amount of water each plant receives (0, 50, 100, or 150 mL per day).
Dependent variable: the height of the bean plants after three weeks (measured in cm).
Controlled variables (any two): type of seed, pot size, soil type, location/light on the windowsill, temperature, watering time of day.
Is the claim justified? No. The investigation only tested bean plants under these specific conditions with four water amounts. Group C (150 mL) grew tallest among the amounts tested, but:
- 150 mL might not be the true best amount - the best value could be between 100 and 150 mL or above 150 mL, and those amounts were never tested.
- The results apply only to bean plants in this classroom setup. "Every plant everywhere" is far beyond the evidence.
A careful conclusion would be: "For these bean plants under these conditions, 150 mL per day produced the tallest growth among the four amounts tested."
2. The students notice that one plant in Group B grew much taller than the other two plants in Group B. What should they do before drawing conclusions, and why does this matter for the reliability of the results?
Read the worked answer
When one plant in a group behaves very differently from the others, the students should check for an error or unusual condition (Was it accidentally given extra water? Was it closer to the window? Was it damaged or measured incorrectly?) and record the data as-is rather than discarding it silently. They should also consider whether that plant is an outlier.
Why it matters: Results are more reliable when repeated trials agree. With only 3 plants per group, one unusual plant can strongly shift the average and make the group look different from what is typical. If the outlier comes from a measurement or treatment mistake, the students should note it and explain their reasoning. If the outlier is real, it suggests the results vary more than expected and the conclusion should be stated with caution - possibly by repeating the investigation with more plants per group.