سؤال 8 — Short Reading 8 — Caution

A small university study asked 80 students to record how long they slept during the week before final examinations. Students who reported seven to eight hours of sleep per night performed better, on average, than those who slept fewer than six hours. The researchers warned that the study does not prove that extra sleep directly causes higher grades. Students who sleep more may also have better study habits or lower stress. Still, the researchers recommend maintaining a regular sleep schedule because adequate sleep is known to support attention and memory.
Why do the researchers avoid claiming that sleep directly caused higher grades?