what does "not significantly different from zero" mean?

Question

"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 first three weeks of the month is, in general, >>>not significantly different from zero<<< and is generally significantly higher than the mean monday return of the last two weeks. Our finding seems to make it more difficult to explain the monday effect."

Answer 1

What it means is that the results from the Mondays in the first 3 weeks of the month don't add anything substantial to the overall results (it's like adding zero). The effect is found only in the last 2 mondays of the month. Thus, they have difficulty explaining the monday effect since it only appears to occur in the last 2 weeks of the month, not evenly spread out across all mondays which you would expect if it were truly a "monday" effect and not a "monday at the end of the month" effect.

Answer 2

Typically means that the number is pretty much zero, but isn't actually nothing. This is mostly found in statistical references, like the one you have described.

As noted in the last line, when the "mean monday return" is not much different from zero, it's difficult to determine what the "monday effect" is.

Answer 3

The probability is high that the difference happened by chance.

Answer 4

as good as not