Normal practice is to treat likert scales as continuous variable even though they are not. As long as there are >=5 options, the bias from discreteness is not large.

I simulated the situation for you. I generated two variables with continuous random data from two normal distributions with a correlation of .50, N=1000. Then I created likert scales of varying levels from the second variable. Then I correlated all these variables with each other.

Correlations of continuous variable 1 with:

continuous2 0.5

likert10 0.482

likert7 0.472

likert5 0.469

likert4 0.432

likert3 0.442

likert2 0.395

So you see, introducing discreteness biases correlations towards zero, but not by much as long as likert is >=5 level. You can correct for the bias by multiplying by the correction factor if desired:

Correction factor:

continuous2 1

likert10 1.037

likert7 1.059

likert5 1.066

likert4 1.157

likert3 1.131

likert2 1.266

Psychologically, if your data does not make sense as an interval scale, i.e. if the difference between options 1-2 is not the same as between options 3-4, then you should use Spearman’s correlation instead of Pearson’s. However, it will rarely make much of a difference.

Here’s the R code.

*#load library*

*library(MASS)*

*#simulate dataset of 2 variables with correlation of .50, N=1000*

*simul.data = mvrnorm(1000, mu = c(0,0), Sigma = matrix(c(1,0.50,0.50,1), ncol = 2), empirical = TRUE)*

*simul.data = as.data.frame(simul.data);colnames(simul.data) = c(“continuous1″,”continuous2″)*

*#divide into bins of equal length*

*simul.data[“likert10″] = as.numeric(cut(unlist(simul.data[2]),breaks=10))*

*simul.data[“likert7″] = as.numeric(cut(unlist(simul.data[2]),breaks=7))*

*simul.data[“likert5″] = as.numeric(cut(unlist(simul.data[2]),breaks=5))*

*simul.data[“likert4″] = as.numeric(cut(unlist(simul.data[2]),breaks=4))*

*simul.data[“likert3″] = as.numeric(cut(unlist(simul.data[2]),breaks=3))*

*simul.data[“likert2″] = as.numeric(cut(unlist(simul.data[2]),breaks=2))*

*#correlations*

*round(cor(simul.data),3)*

I (the author of the above review, Sebastian Sauer), declare that I have no competing interests.