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I know this will vary from journal to journal, but I'm working on my first publication and have had little guidance, so any personal experiences would be helpful to me.

When you present your p-values for highly significant data, where do you stop? For example, I have some p-values that are equal to 7 * 10^(-14) (or 7E-14, or 0.00000000000007). In this situation, where do you make the cut-off? Would you just say p < 0.00001? If so, an additional question arises - if I have two tests that are highly significant but are different from each other, say one has p = 2*10^(-10) and the other is p = 5*10^(-30), are they both reported as p < 0.0001 (or whatever the cut-off is)? Does it matter that one is so much smaller (and therefore more significant) than the other?

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  • $\begingroup$ I think this would be better suited to the statistics SE. Although the numeric difference between 2E-10 and 5E-30 is very large, I suspect that the practical difference between them is probably negligible. Note that the interpretation of p-values is rather subtle, and the level of significance should be chosen prior to the analysis (which essentially indicates roughly where the cut-off lies). $\endgroup$ – Dikran Marsupial Mar 12 '15 at 15:55
  • $\begingroup$ I think it is suitable here since it is a style question and not a stats question. $\endgroup$ – StrongBad Mar 12 '15 at 15:57
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    $\begingroup$ I'm voting to close this question as off-topic because answers to this question hugely depends on the level of accuracy your research topic needs. Some need more and some need less. $\endgroup$ – Enthusiastic Student Mar 12 '15 at 17:53
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    $\begingroup$ See here, one answer of which already gives the APA-style answer and another answer discusses issues of meaningfulness of extremely small p-values and which is relevant to your question at the end. While it's not an exact duplicate (and so perhaps wouldn't close as one) I believe your questions are addressed there. $\endgroup$ – Glen_b -Reinstate Monica Mar 12 '15 at 23:10
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    $\begingroup$ @Glen_b: +1 to your comment, but I think it is a duplicate and vote to close it as such. $\endgroup$ – amoeba says Reinstate Monica Mar 13 '15 at 0:07
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For APA 6 Style

Decimal fractions—new guidelines for reporting of p values to two or three decimal places. (However, p values less than p<.001 should be reported as p<.001).

I believe in APA 5 Style you reported the exact p value when p is greater than your magic bar (e.g., .5) and p < .5 when p was less than the magic bar, unless it was less than .001 in which case you report p < .001.

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