The Monday effect on stock prices. Puzzling but true: stocks tend to go down on Mondays. There is no convincing explanation for this fact. A recent study looked at this "Monday effect" in more detail, using data on the daily returns of stocks on several U.S. exchanges over a 30-year period. Here are some of the ?ndings:

To summarize, our results indicate that the well- known Monday effect is caused largely by the Mondays of the last two weeks of the month. The mean Monday return of the ?rst three weeks of the month is, in general, not signi?cantly different from zero and is generally signi?cantly higher than the mean Monday return of the last two weeks. Our ?nding seems to make it more dif?cult to explain the Monday effect.

A friend thinks that "signi?cantly" in this article has its plain English meaning, roughly "I think this is important." Explain in simple language what "signi?cantly higher" and "not signi?cantly different from zero" actually tell us here.