Spearman's rho is based on ranks. It is pretty much the same calculations as Pearson's r, except where Pearson's uses the actual values of both variables, Spearman's correlates the rankings of those variables.

Therefore Pearson's is appropriate for interval or ratio level parametric data, where the intervals are meaningful and can be uniformly measured (ie measurements like height, score on IQ test, etc). If the data is non-parametric of ordinal level only (ie responses to a 5 point likert scale of satisfaction) the Spearman's should be used.

Therefore Pearson's is appropriate for interval or ratio level parametric data, where the intervals are meaningful and can be uniformly measured (ie measurements like height, score on IQ test, etc). If the data is non-parametric of ordinal level only (ie responses to a 5 point likert scale of satisfaction) the Spearman's should be used.