# p-values of Mann-Whitney U test identical for raw and log-transformed data

I am new to Stats and came across this problem while running my analyses on SPSS which I cannot explain.

How is it that even after transforming my data by logging it, it still has the same p-value as the raw data set that was not transformed?

Sorry for sounding really ignorant here! Thank you in advance for your help.

Taking logs (of positive values), or indeed, any other monotonic transformation of the values (like taking $-1/x$ with positive data values, or cubing a set of values) will not change the order (and hence the ranks assigned to observations), only their relative spacing, so the statistic won't be affected. (Even a monotonic decreasing transformation won't change the p-value of the two-tailed test.)