The sign test is used to compare the medians of paired or matched observations. It is an alternative to the paired-samples t-test (Chapter @ref(t-test)) and the Wilcoxon signed-rank test (Chapter @ref(wilcoxon-test)) in the situation, where the distribution of differences between paired data values is neither normal (in t-test) nor symmetrical (in Wilcoxon test).
Note that, the sign test does not make any assumptions about the data distributions. However, it will most likely be less powerful compared to the Wilcoxon test and the t-test.
Therefore, if the distribution of the differences between the two paired groups is symmetrical in shape, you could consider using the more powerful Wilcoxon signed-rank test instead of the sign test.
In this chapter, you will learn how to compute paired-samples sign test using the R function
sign_test() [rstatix package].
Make sure that you have installed the following R packages:
tidyversefor data manipulation and visualization
ggpubrfor creating easily publication ready plots
rstatixprovides pipe-friendly R functions for easy statistical analyses
datarium: contains required datasets for this chapter
Start by loading the following required packages:
library(tidyverse) library(rstatix) library(ggpubr)
Here, we’ll use a demo dataset
mice2 [datarium package], which contains the weight of 10 mice before and after the treatment.
# Wide data data("mice2", package = "datarium") head(mice2, 3)
## id before after ## 1 1 187 430 ## 2 2 194 404 ## 3 3 232 406
# Transform into long data: # gather the before and after values in the same column mice2.long <- mice2 %>% gather(key = "group", value = "weight", before, after) head(mice2.long, 3)
## id group weight ## 1 1 before 187 ## 2 2 before 194 ## 3 3 before 232
The paired-samples sign test evaluates whether the median of paired differences is statistically significantly different to 0.
- Null hypotheses, H0: median of the paired differences = 0
- Alternative hypotheses, Ha: median of the paired differences is different to 0
Compute some summary statistics by groups: median and interquartile range (IQR).
mice2.long %>% group_by(group) %>% get_summary_stats(weight, type = "median_iqr")
## # A tibble: 2 x 5 ## group variable n median iqr ## <chr> <chr> <dbl> <dbl> <dbl> ## 1 after weight 10 405 28.3 ## 2 before weight 10 197. 19.2
bxp <- ggpaired(mice2.long, x = "group", y = "weight", order = c("before", "after"), ylab = "Weight", xlab = "Groups") bxp
Question : Is there any significant changes in the weights of mice after treatment?
stat.test <- mice2.long %>% sign_test(weight ~ group) %>% add_significance() stat.test
## # A tibble: 1 x 9 ## .y. group1 group2 n1 n2 statistic df p p.signif ## <chr> <chr> <chr> <int> <int> <dbl> <dbl> <dbl> <chr> ## 1 weight after before 10 10 10 10 0.00195 **
We could report the results as follow:
The median weight of the mice before treatment is significantly different from the median weight after treatment using sign test, p-value = 0.002.
stat.test <- stat.test %>% add_xy_position(x = "group") bxp + stat_pvalue_manual(stat.test, tip.length = 0) + labs( subtitle = get_test_label(stat.test, detailed= TRUE) )
This chapter describes how to compute and report the Sign test in R.
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