What is the difference between the Mann-Whitney U-test and Wilcoxon Signed ranks test? I know that these two methods are testing whether there is a difference of two samples, but in which situation can I use Mann-Whitney and when Wilcoxon can be used?
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Maybe I can add some examples to make the question more clear. a)Weight measurements were taken on 30 individuals before and after they each completed the same diet plan. An investigator was interested in investigating if there was a population median change in weight after the diet. – alex.wang Nov 24 '21 at 18:13
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b)Measurements of water quality were recorded on the same 20 locations in a lake before and after the removal of a sewage treatment works near the lake. An investigator was interested in investigating if there was a change in the population mean water quality after the removal of the sewage treatment works. – alex.wang Nov 24 '21 at 18:15