A study that gives one group a treatment and another a comparison is an experiment. Check who decided which group got the treatment: random assignment supports a causal claim, while plants choosing their own groups would show only an association. Then check where the plants came from. Random assignment does not extend a result beyond the population represented in the study.
Hints
- Hint 1
Random assignment means chance decides which plants receive a treatment. That helps separate the treatment's effect from differences the plants might have had beforehand. Who decided which seedlings received fertilizer?
- Hint 2
A population is the full group a conclusion describes. Check which plants could have entered this experiment. Could basil plants growing in other greenhouses have been selected from these seedlings?
Step-by-step
Check cause, then check reach
Step 1Identify how the groups were formed
No Desmos needed. The question is about study design, not arithmetic.
The botanist randomly selects which 120 seedlings get fertilizer; the other 120 get water. This is random assignment: chance, rather than a difference between plants, determines their groups. The water-only group gives a comparison, and random assignment lets that comparison test whether the fertilizer caused a difference.
- Step 2
Interpret the measured difference
The fertilizer group has a statistically significantly greater mean mass. The mean is the group average; the result doesn't say every fertilized plant became heavier. Together with random assignment, this difference supports an increase in average mass caused by the fertilizer.
- Step 3
Keep the conclusion within the studied group
All 240 seedlings came from one greenhouse. None came from other greenhouses or other basil varieties, so the result shouldn't be extended to all plants of this variety or to all basil plants. Random assignment supports cause, but the plants' source limits how far the conclusion reaches. For the basil plants in this greenhouse, weekly fertilizer causes an increase in mean plant mass. Choice A.