​CONTROL GROUP

​In the past, students in Mrs. Garza’s class have conducted investigations growing wheatgrass seeds. They have observed that wheatgrass grows well in a four‐inch pot containing 200 grams of potting mix. Plants do well in the sunlight on the counter. Six milliliters of water per day seems to be the best amount of water. Student in the class are wondering is the plants has to get six milliliters of water all at once, or if the six milliliters of water can be given to the plant as 3 doses of 2 milliliters each. The class prepares twenty‐four new plants for growth, as they have in the past. All 24 plants are placed on the counter, where they will all get the same amount of sunlight and air. Twelve of the plant pots are labeled with yellow tape, and will receive 6 ml of water once a day. Twelve of the plants are labeled with red tape, and will receive 2 ml of water 3 times a day, for a total of 6 ml per day.

​In this setup, the variable being tested is not the amount of water: all plants are received 6ml of water every day. The variable being tested is the number of times the plants are watered each day. The yellow group is getting one watering of six milliliters, and the red group is getting watered 3 times a day with 2 ml of water.

In the experiment, the yellow group is considered the control group. The number of times these plants are watered is not changing from what previous investigations have shown to work.

The variable being tested is the number of times the plants are watered. The red group is being watered 3 times a day.

In an experiment, the control group is the group that is not testing a variable: nothing is being changed for the control group. The variable is the thing being tested, or changed, in order to provide empirical data to draw conclusions about the experiment.

​Is there a “variable group?” There is not: in the experiment, there is only the control group and the variable.

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Remember: the variable is the thing being changed to test a question, not a group.

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