So far, we have been talking about causes and effects in qualitative terms. An effect either occurs or does not occur; a factor is either present or absent. However, both sides of this equation can vary quantitatively as well.

Concomitant variation is the method in which a quantitative change in the effect is associated with quantitative changes in a given factor.

Example:

If you car makes a funny noise when you accelerate, you might take your foot off the pedal and see whether the noise goes away. As we saw in the earlier example, that would be the method of difference. But you might also vary the pressure on the pedal to see whether the noise varies in intensity accordingly. That would be the method of concomitant variation.