Hi. I have data from a tiling microarray experiment. The column I am interested in contains the p-values for sex bias. They are supposedly all significant (only the significant ones were included). However, instead of being in the usual decimal notation of 0 to 1 (or 0 to 0.05 in this case), they range from -300 to -1.5 and 1.5 to 300. Does anyone know how to convert this? The person who originally generated this data is not easy to get a hold of. Thanks.
Since 10^(-1.5) = 0.03, it is likely that the P-values have just been log transformed. The sign probably refers to direction of the bias (eg. +'ve for male, -'ve for female). To convert back, ignore the sign and just compute: Pvalue = 10^(-1 * score).
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ethan.kaufman
This is done when data storage precision is an issue. If you only get eight characters to write each number, both 10^-100 and 10^-150 look like exactly zero when truncated.
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karl.stamm
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Are you sure it's a p-value? Could it be an F-test statistic? Maybe try to reproduce the calculation for one of the bits of data, then you could check if it's log transformed suggested by the answer?
It's unlikely to be anything other than a p-value, as the column heading is "sex.P". My supervisor just remembers that the 2.0 corresponds to 0.01 and the 1.5 to 0.05. I can reproduce the first (10^-2 =0.01), but not the second.
Oh, well. It will get figured out eventually.