Limit of p value from multcompare?

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Eric Zhang 2016-6-27

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/292507-limit-of-p-value-from-multcompare

评论： the cyclist 2016-6-29

(I'm having exactly the same question as this one on Stack Exchange, so I'm quoting it verbatim as follows)

It seems to me that MATLAB multcompare cannot output any p value (for pairwise comparison) smaller than 9.56061674273201e-10.

To confirm this, I did

y = [ones(100,1) 2*ones(100,1) 3*ones(100,1)];
[p,tbl,stats] = anova1(y);
[c,~,~,gnames] = multcompare(stats);
p = c(:,6)

and got p values for the three pairwise comparisons

p =
        9.56061674273201e-10
        9.56061674273201e-10
        9.56061674273201e-10

Why is there this limit in MATLAB? How should I report my p values then? It looks strange to report exactly same values for several different tests.

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Eric Zhang 2016-6-28

编辑：Eric Zhang 2016-6-28

@Brendan Hamm

Many thanks for the reply! Very interested in how exactly this rounding error occurs (or this limit is reached). Greatly appreciate it if you could elaborate more? Thanks a lot!

the cyclist 2016-6-29

I've attached a couple references that I've found useful.

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