Yes the P values being reported by different software are like any other calculated numbers with decimal places: They are actually rounded values, not real values. In some software packages, you can configure the level of rounding the P value (to three, four, or sometimes to more than four decimal places), but in some, it is not possible. Anyway, a P = 0.0000 can be actually P = 0.0004888888, = 0.0000000002, or = 0.0000000000000000000000000000000000000000001. All of these are reported as 0.0000 (as the rounded P value). In SPSS it is possible to check the "real" not-rounded P value, but in Minitab it is not possible.

However, another point here is that Mann-Whitney does not compare medians, actually (although it is widely believed so). It actually compares the dispersions around medians. A search within this forum, especially in posts sent by CowboyBear would give you more details.

Two other points: 1. as far as I know, Minitab reports only three places of decimal, so I wonder which version of Minitab you use which shows 4 decimal places in P value?

2. A P value of 0.000049 is still quite sufficient to reject your null hypothesis, so I don't understand why it baffled you in the first place?

However the difference seems to be zero - which is the same as saying that the the two are the same.

I think you are thinking that the P value refers to the difference between medians, no? So you were baffled since you saw the difference (that you though is shown by P value) is zero, but you are now relieved since you see the difference between the medians (shown by P values) is not actually zero (and is something like 0.00004 for example).

If I am right and you think P value is the difference between medians, you should study the notion of P value first, since you seem to be on a wrong path right now. But fear not!

That is an interesting matter and there are many high-quality threads already available on P value (as well as experts willing to help you).

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The fact that the p-value AND THE diference were both zero at first sight.

Aha I see

but then you saw that P is not actually zero, and was relieved to see that a zero difference between medians led to a P significant at that level? Didn't you think that a difference as small as 0.0000 is least likely to accompany a P value not about 1.0?

And let me ask again, which Minitab version reports four decimal places? Could you please let me know how you managed to extract four decimal places out of that software, because I could use your method.

Yeah I found it, thanks.

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lol yeah I was thinking you are the original poster lol

Nice to meet you!