Question: RankProd: Why are p-values=0?
0
3.9 years ago by
United States
brilladine0 wrote:

Hello,

I am trying to make a volcano plot using the outputs provided by RankProd. Many of my genes have a p-value = 0, according to RankProd, which makes plotting the -log10(p-value) difficult to plot.

Are p-values = 0 the product of an error, or an underflow issue? What's the best way to address such underflow issues?

Thank you.

rankprod p-value • 1.3k views
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modified 3.9 years ago by James W. MacDonald50k • written 3.9 years ago by brilladine0
Answer: RankProd: Why are p-values=0?
4
3.9 years ago by
United States
James W. MacDonald50k wrote:

I don't think it's an underflow issue. Instead, the p-values appear to be computed as

# permutations more extreme than observed/# permutations

And if you don't have any permuted statistics that are as or more extreme than your observed statistic, this ends up being

0/# permutations

There are arguments for doing something like

# permutations more extreme + 1 / # permutations

Which is what the limma package does, where you always count the observed result as one of the permuted observations. With 1000 permutations, it's a difference between 0 and 0.001, which is probably the basis of the argument - neither one is likely to be correct (the observed statistic is only expected to be part of the null distribution if there truly are no differences), but 0.001 is probably a bit more interpretable than a p == 0, so there you go.

ADD COMMENTlink written 3.9 years ago by James W. MacDonald50k
1

Also, a related paper by B. Phipson and G. Smyth (Stat Appl Genet Mol Biol. 2010): Permutation P-values should never be zero: calculating exact P-values when permutations are randomly drawn

ADD REPLYlink written 3.9 years ago by Diego Diez730
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