Contents
What does a p value mean for the Wilcoxon test?
Whether exact or approximate, p-values do not tell us anything about how different these distributions are. For the Wilcoxon test, a p-value is the probability of getting a test statistic as large or larger assuming both distributions are the same.
When is the Wilcoxon rank sum test cannot be used?
The impact of ties means the Wilcoxon rank sum distribution cannot be used to calculate exact p-values. If ties occur in our data and we have fewer than 50 observations, the wilcox.test function returns a normal approximated p-value along with a warning message that says “cannot compute exact p-value with ties”.
Is the null hypothesis equal to the Wilcoxon test?
Whereas the null hypothesis of the two-sample t test is equal means, the null hypothesis of the Wilcoxon test is usually taken as equal medians.
When to use normal approximation or continuity correction?
When we use the normal approximation the phrase “with continuity correction” is added to the name of the test. A continuity correction is an adjustment that is made when a discrete distribution is approximated by a continuous distribution. The normal approximation is very good and computationally faster for samples larger than 50.
What is the p value of the rank sum test?
p = ranksum(x,y) returns the p-value of a two-sided Wilcoxon rank sum test. ranksum tests the null hypothesis that data in x and y are samples from continuous distributions with equal medians, against the alternative that they are not. The test assumes that the two samples are independent. x and y can have different lengths.
What are the results of the Wilcoxon rank sum test?
This test is equivalent to a Mann-Whitney U-test. [p,h] = ranksum (x,y) also returns a logical value indicating the test decision. The result h = 1 indicates a rejection of the null hypothesis, and h = 0 indicates a failure to reject the null hypothesis at the 5% significance level.
Which is the rank sum test for X and Y?
p = ranksum (x,y) returns the p -value of a two-sided Wilcoxon rank sum test. ranksum tests the null hypothesis that data in x and y are samples from continuous distributions with equal medians, against the alternative that they are not. The test assumes that the two samples are independent. x and y can have different lengths.