A biology student wanted to determine if using fertilizer would promote plant growth. The student randomly selected 20 plants and randomly assigned 10 of them to a large square container and the other 10 to a different large square container. Both containers had the same amount of soil and received the same amount of water and light. One container also received fertilizer and the other container did not. Is this a valid experimental design?

The fertilizer was applied to a whole container, so the container—not each individual plant—was the experimental unit. There was only one container with fertilizer and one without, so the experiment had no replication at the level where the treatment was applied. Differences in the containers themselves could affect plant growth, making it impossible to tell whether fertilizer caused any difference.

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