Show that students who study more hours receive higher grade

Question

Suppose studies show that students who study more hours receive higher grades.  Is there any relationship which guarantees that any particular student who studies longer will get higher grades?

Answer 1

No, not necessarily, because cause and effect are not clear.  The observed relationship could largely reflect the fact that smarter students study more hours than do less able students.  A less talented student therefore may not get better grades by studying longer where as the extent suggested by the graph at least.