Wilcoxon signed rank test

July 29, 2009
By

(This article was first published on Statistic on aiR, and kindly contributed to R-bloggers)

Non-parametric statistical hypothesis test, for the comparison of the means between 2 paired samples

The mayor of a city wants to see if pollution levels are reduced by closing the streets to the car traffic. This is measured by the rate of pollution every 60 minutes (8am 22pm: total of 15 measurements) in a day when traffic is open, and in a day of closure to traffic. Here the values of air pollution:

With traffic: 214, 159, 169, 202, 103, 119, 200, 109, 132, 142, 194, 104, 219, 119, 234
Without traffic: 159, 135, 141, 101, 102, 168, 62, 167, 174, 159, 66, 118, 181, 171, 112


It is clear that the two groups are paired, because there is a bond between the readings, consisting in the fact that we are considering the same city (with its peculiarities weather, ventilation, etc.) albeit in two different days. Not being able to assume a Gaussian distribution for the values recorded, we must proceed with a non-parametric test, the Wilcoxon signed rank test.


a <- c(214, 159, 169, 202, 103, 119, 200, 109, 132, 142, 194, 104, 219, 119, 234)
b <- c(159, 135, 141, 101, 102, 168, 62, 167, 174, 159, 66, 118, 181, 171, 112)

wilcox.test(a,b, paired=TRUE)

Wilcoxon signed rank test

data: a and b
V = 80, p-value = 0.2769
alternative hypothesis: true location shift is not equal to 0



Since the p-value is greater than 0.05, we conclude that the means have remained essentially unchanged (we accept the null hypothesis H0), then blocking traffic for a single day did not lead to any improvement in terms of pollution of the city.
The value V = 80 corresponds to the sum of ranks assigned to the differences with positive sign. We can manually calculate the sum of ranks assigned to the differences with positive sign, and the sum of ranks assigned to the differences with negative sign, to compare this interval with the interval tabulated on the tables of Wilcoxon for paired samples, and confirm our statistic decision. Here's how to calculate the two sums.


a <- c(214, 159, 169, 202, 103, 119, 200, 109, 132, 142, 194, 104, 219, 119, 234)
b <- c(159, 135, 141, 101, 102, 168, 62, 167, 174, 159, 66, 118, 181, 171, 112)

diff <- c(a - b) #calculating the vector containing the differences
diff <- diff[ diff!=0 ] #delete all differences equal to zero
diff.rank <- rank(abs(diff)) #check the ranks of the differences, taken in absolute
diff.rank.sign <- diff.rank * sign(diff) #check the sign to the ranks, recalling the signs of the values of the differences
ranks.pos <- sum(diff.rank.sign[diff.rank.sign > 0]) #calculating the sum of ranks assigned to the differences as a positive, ie greater than zero
ranks.neg <- -sum(diff.rank.sign[diff.rank.sign < 0]) #calculating the sum of ranks assigned to the differences as a negative, ie less than zero
ranks.pos #it is the value V of the wilcoxon signed rank test
[1] 80
ranks.neg
[1] 40


The computed interval is (40, 80). The tabulated interval on Wilcoxon paired samples tables, with 15 differences, is (25, 95). Since the calculated interval is contained in the tabulated, we accept the null hypothesis H0 of equality of the means. As predicted by the p-value, closing roads to traffic did not bring any improvement in terms of rate of pollution.

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