Control Group
Take identical growing plants, randomly assign them to two groups, and give fertilizer to one of the groups. If there are differences between the fertilized plant group and the unfertilized "control" group, these differences may be due to the fertilizer.
Source
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"Starr 011107-0010 Argyroxiphium sandwicense subsp.
macrocephalum."
http://en.wikipedia.org/wiki/File:Starr_011107-0010_Argyroxiphium_sandwicense_subsp._macrocephalum.jpg
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