Independent and dependent variables are mathematical tools used in an experiment to keep track of what's going on.
They allow you to maintain control over your experiment in a quantitative way. That is, using them, you will be able to measure your results and draw accurate conclusions.
Growing plants for an experiment outside may seem like a good idea, but watch out for plant tropisms that you didn't expect due to things like the crack in this fence.
Independent and dependent variables are related to one another. The Independent part is what you, the experimenter, changes or enacts in order to do your experiment. The dependent variable is what changes when the independent variable changes - the dependent variable depends on the outcome of the independent variable.
For instance: if you were measuring the growth rate of plants under full sunlight for 8 hours a day versus plants that only have 4 hours of full sunlight per day, the amount of time per day of full sunlight would be the independent variable - the variable that you control. The growth rate of the plants would be a dependent variable.
A dependent variable? Yes, there can be more than one dependent variable. In our example, for instance, the growth rate of the plants might be one dependent variable and the overall height of the plants might be another dependent variable. Both of these variables dependupon the independent variable.
When we talk about independent and dependent variables, we mentioned that you can have more than one dependent variable. Can you have more than one independent variable? No, there should be only one independent variable for any valid experiment (advanced forms of research do allow for more than one independent variable but unless you are doing this kind of research, you should probably stick to one.)
If you have more than independent variable in your experiment (more than one variable that is affecting your dependent variables) that probably indicates that you have not properly identified and dealt with your controlled variable.
Breaking this down further, we summarize independent and dependent variables in this way:
Controlled variables must be carefully monitored and kept equal in your experiments - otherwise they could mess up your experiment by making your results false or unreliable.
When discussing independent and dependent variables, controlled variables are anything else that could have an influence on your project. For instance, in our example, the amount of water that each plant received is not our independent variable.
But could the amount of water affect the growth speed or size of our plants? Absolutely. It is, therefore, considered a control variable. You would, therefore, have to make sure that each plant got the same amount of water as every other plant in the experiment - in this way you eliminate the amount of water each plant receives (a control variable) as a possible source of difference between your plants (your different experimental subjects.)
How many control variables are there per experiment? That depends upon the experiment - but there could be many. In our example, some possible control variables would be: water amount, humidity of the air, air temperature, soil quality, vibration, fauna (bugs), and many others.
Your job as an experimenter is to make sure that all (or nearly all) of these control variables are the same for each of your experimental subjects (plants in this case.)