Aresearcher is conducting a study on contact lenses. she finds that in a sample of contact lens-wearing adults, those who wear their contact lenses for longer periods of time have more irritation in their eyes. those who wear their contact lenses for only a few hours a day have less irritation. however, those who wear their contacts for longer times also tend to spend long hours at the office. as explanations for having more irritation in the eyes, the variables "length of time contacts are worn each day" and "length of time spent at the office per day" are variables.