dplyr - Performing Mann-Whitney (Wilcox) Test on Grouped Data in R Studio?

Question

I have a dataset that measures the abundance of macroinvertebrates from multiple sample sites. I wish to compare results from the most recent years of sampling with results from all previous years of sampling at the same sites.

My data looks like this:

# A tibble: 6 x 5
  basin                sitecode sampleid metric       value
  <fct>                <chr>       <int> <chr>        <dbl>
1 arctic coast islands HUSK1       13482 s_abundance1  5312
2 arctic coast islands HUSK1       13482 s_abundance2    NA
3 arctic coast islands NOEL1       13488 s_abundance1   616
4 arctic coast islands NOEL1       13488 s_abundance2    NA
5 arctic coast islands RPR070       6815 s_abundance1    NA
6 arctic coast islands RPR070       6815 s_abundance2  697
> 

s_abundance1 stands for site abundance for most recent site and s_abundance2 stands for site abundance at previously sampled site(s)

The entire dataset is about 4000 rows and is comprised of sample data for many different drainage basins.

I would like to perform a mann-whitney u test comparing s_abundance1 against s_abundance2, but grouped by basin in a single output

The code I have been using is:

abund_results %>%  
+     group_by(basin) %>%
+     summarise(tidy(wilcox.test(abund_results$value ~ abund_results$metric, data = .)))

It seems to work, except that all of the p-values come out exactly the same. Here is the output:

abund_results %>%  
+     group_by(basin) %>%
+     summarise(tidy(wilcox.test(abund_results$value ~ abund_results$metric, data = .)))
`summarise()` ungrouping output (override with `.groups` argument)
# A tibble: 17 x 5
   basin                statistic   p.value method                                 alternative
   <fct>                    <dbl>     <dbl> <chr>                                  <chr>      
 1 arctic coast islands   181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 2 columbia               181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 3 fraser lower mainla…   181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 4 great lakes            181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 5 lower mackenzie        181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 6 lower saskatchewan-…   181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  
 7 maritime coastal       181204. 5.82e-108 Wilcoxon rank sum test with continuit… two.sided  

What do I need to change to get different results for each basin?

Answer 1

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