correlation of two ordinal variables in R

Question

I want to test the spearman correlation of two ordinal variables.

x=c(1,2,3)
y=c(4,3,6)
x=ordered(x)
y=ordered(y)
cor(x,y,methods="spearman")

I always get "Error in cor(x, y) : 'x' must be numeric"

what's the correct way to do this?

Answer 1

Two methods:

1. use as.numeric.

   x=c(1,2,3)
   y=c(4,3,6)
   x=ordered(x)
   y=ordered(y)
   cor(as.numeric(x), as.numeric(y), method="spearman")
   [1] 0.5
   

Note that this is not treating x and y simply as continuous numbers. It is treating them as ranks.

as.numeric(y)
[1] 2 1 3

This method will allow you to ignore NA values.

x=c(1,2,3, NA)
y=c(4,3,6, 7)
x=ordered(x)
y=ordered(y)
cor(as.numeric(x), as.numeric(y), 
    method="spearman", use="pairwise.complete.obs")
[1] 0.5

2. You can use the package pspearman which will handle the ordered factor.

   x=c(1,2,3)
   y=c(4,3,6)
   x=ordered(x)
   y=ordered(y)
   
   library(pspearman)
   spearman.test(x,y)
   
   
   Spearman's rank correlation rho
   
   data:  x and y
   S = 2, p-value = 1
   alternative hypothesis: true rho is not equal to 0
   sample estimates:
   rho 
   0.5 
   

Or if you want to reduce some of the output, you could use:

spearman.test(x,y)$estimate
rho 
0.5