In the last lectures I talked about Pearson’s r, which measures the relationship between two continuous (interval or ratio scale) variables.

Spearman Correlation |
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The Spearman correlation is used when:
1. Measuring the relationship between two ordinal variables. 2. Measuring the relationship between two variables that are related, but not linearly. |

Below is an example of some data that is related in a non-linear fashion. For this, we would use the Spearman correlation:

Figure 1. |
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Let's calculate the Spearman correlation for the following data set:

Figure 2. |
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To calculate the Spearman correlation, we must first rank the scores:

Figure 3. |
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We then calculate the correlation using these new ranks:

Figure 4. |
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We find an r of -1.00, meaning that our data has a negative relationship. As x increases, y decreases. As x decreases, y increases.