What is a controlled variable in a science experiment?

I know an independent variable and dependent variable but have trouble understanding a controlled variable?What is a controlled variable in a science experiment?
It's something kept constant in all the setups of the experiment.





For example:


If the experiment is about how the intensity of light affects plant growth, the controlled variables would be things like the type of plant, the amount of soil used, the amount of water used and the colour of the light.





So basically, it's just variables that are not changed in the experiment.What is a controlled variable in a science experiment?
There are different ways of defining terms depending on who is teaching you about experiments and it can be confusing.





One way to think about the inputs to any experiment is to consider 3 types of inputs...


1) There are factors you set at specific levels, e.g., low, medium, and high or on/off. These are the Controlled variables, the things you are studying.


2) There are other variables that you are probably ';controlling'; even if you are not thinking about it. These would be things like gravity, if you are doing the experiment on earth, the force of gravity is 1 G or maybe you are doing the experiment in an air conditioned lab so room temperature is pretty close to 20 C and the relative humidity is low or maybe you are blending your ingredients for each experimental run in a kitchen blender on high power. All of these can be called ';SOP'; for standard operating procedure or ';something on paper'; because, if you repeat the experiment, you are pretty sure gravity is still 1 G or high power on the blender is still high power.


3) Then you have everything else in the universe that might be changing during your experiment. These factors can be called noise. One way to check to see if noise has influenced your experiement to randomize the way you run the experiments. For example, if you have 10 runs at factor A low and 10 at medium and 10 at high and if you run them in sequence and, in the unlikely event that a noise factor does influence the results, there is no way for you to know if it was A or that noise which created what you saw.





The outputs of your experiment are what ever they are. In addition to the results that you are measuring, there are other outputs you may not think of. These include things like the elapsed time required to complete the experimental run or the time of day when the samples were actually measured.





hope this helps
It is an independent variable that does change with respect to conditions attached to an experiment.