A student is testing the effect of a new fertilizer on tomato growth. She plants one tomato in garden soil with no fertilizer and plants the other tomato in garden soil with fertilizer added.

There is only one tomato in each condition: one without fertilizer and one with fertilizer. That means the sample size is one tomato per group, so differences in growth could be due to individual variation rather than the fertilizer. The tomato receiving no fertilizer does serve as a control group.

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