Whats the difference between a constant variable and a control in science?

A costant varible is one that does not change, a control does. For example the equation for the area of a circle would be A=pie*R^2. Whereas pie would be the constant and R or the radiaus would be the control. Same thing applies to experiments as it does equations.Whats the difference between a constant variable and a control in science?
In experimental design, you're trying to determine the effect of some variable that you're manipulating. So, for example, if your null hypothesis (what you're trying to DISPROVE, thus supporting the alternative hypothesis) might be that ';Consumption of alcohol has no impact on reaction time';, and you might get a heap of people and break them into 3 groups; one group has no alcohol, the second group may have some defined level eg 1 standard drink in an hour, and the third group may have 2 standard drinks in an hour.





The ';control'; group would be those who have no alcohol - it's kind of like the ';default'; state in the absence of the factor that you're trying to manipulate.





A constant variable would be some other factor that can vary and may impact on reaction time, but you ensure that it's the same between the groups to ensure that you don't have other effects on reaction time that you're not wanting. So, in this example, other variables that may impact on reaction time would include whether people are smokers or non-smokers, and whether they're on any medication, so you'd ensure that all participants were the same for these variables (eg all non-smokers and not on medication) so that you don't have variation in reaction time that could be explained by these factors rather than alcohol consumption. If you didn't control for these variables and effects on reaction time were introduced by these differences, they would be known as ';confounding variables';. But those that you ensure are constant between groups are ';constant variables';.





Hope this helps!