Why can Likert-scale responses be analyzed with Spearman rank-order correlation?

Likert-scale responses are analyzed using the Spearman rank-order correlation because they fall within the ordinal level of measurement.1

In statistics, data are classified by their scale of measurement. Likert scales possess directionality—meaning respondents understand that "strongly agree" is more positive than "agree"—but the intervals between these categories cannot be presumed to be equal. Because Likert-derived data are not truly interval in nature, they are considered ordinal or ranked data.12

The Spearman rank-order correlation coefficient is specifically designed to measure the strength of a monotonic association between two variables measured on an ordinal or ranked scale. By using this method, researchers can appropriately analyze the relationship between variables without making the potentially invalid assumption that the data are at the interval level, which would be required for parametric tests like Pearson’s product-moment correlation.21