One of the assumptions for Spearman correlation is that there is a monotonic relationship between the two variables. I've created a scatterplot for two Likert item variables, but it seems impossible to get the desired monotonic plot. Must the monotonicity assumption be met for me to calculate the Spearman correlation?
My second question is, can I correlate respondent's score (scale) with a Likert item or any one variable that is nominal. In this case should I rank the score first in SPSS by creating a new column for rank score or will SPSS do it for me?
Best Answer
Spearman's rank correlation only captures monotonic relationships. If the relationship between your two variables is not monotonic, then rank correlation is inappropriate. If the variables were continuous, there are things you could do such as transform the variables or look at splines, but that seems wrong with 5 point scales. It might be best to go to a non-ordinal measure of relationship, such as chi-square.
For your second question, if it's just about how to do something in SPSS it is off topic here. However, if score is a scale variable then you shouldn't need to rank it at all.
Note that, as @NickCox points out in a comment below, that a monotonic relation is not an assumption of either Pearson's or Spearman's correlation; however, as I say above, if you are trying to capture a non-monotonic relationship, correlation is not the right tool.