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What's the difference between a control, constant, dependent and an independent variable?
Controlled variable ##rArr## have nothing done to it. Constant variable ##rArr## kept the same Dependent variable ##rArr## you observed/measured Independent variable ##rArr## changed
Lets take this scenario, you want to find out how the volume of water watered can affect the growth of the bean sprouts .
So you have 2 pots for this experiment, Pot A and Pot B. With Pot A having a usual volume of ##50ml## watered every time, and Pot B having a higher than usual volume of ##100ml## watered every time.
Independent Variable: The variable that you changed.
 In this case is the volume of water watered .
Dependent Variable: The variable that you observed/measured.
 In this case is the height of the bean sprout .
Constant Variable: The variable that is kept the same throughout the experiment.

In this case are:

Type of Pots used
 Number of Seeds planted
 Temperature of the surroundings
 Time at which the plants are watered
Of course, there are other variables that should be kept constant, the above 4 are just examples.
Controlled Variable: The variable that have nothing done to it.
 In this case it refers to Pot A , which is the usual pot, no independent variable existed as the volume of water have no change (usual ##50ml##).