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Khoury et al¹ report significant correlations between peripheral blood T-cell activation markers and disease activity in multiple sclerosis (MS). Indeed, they find a number of statistically significant correlations between changes in percentages of T cells expressing CD25⁺ or CD4⁺ and magnetic resonance imaging measures of disease activity and even disability measurements. However, in placing emphasis on the statistical significance (P value) of the test result, they make the classic error of ignoring the actual magnitude of the observed correlation. Most of their conclusions about relationships between T-cell activation and disease activity are based on correlations with r values between 0.16 and 0.33 (with 0.0 representing no correlation at all and 1.0 a perfect correlation). To put the magnitude of the observed correlations into perspective, the observed relationship can only explain (r²) 2.5% to 11% of the variability in the selected parameters!² Conversely, other parameters must explain the . . . [Full Text of this Article]