You are conducting research on the effect of social media use on depression. You think the effect of social media use on depression would depend on a third variable – the amount of face-to-face social interaction. People who have a low level of face-to-face social interaction would likely experience depression after social media exposure, but people who have a high level of face-to-face social interaction would not likely experience depression after social media exposure. In this case, the third variable – face-to-face social interaction – is:
