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Relationship between the NDI and TL when modeled as a spline effect. The association between NDI and TL was statistically significant when modeled as a spline effect (P-value = 0.0007) and when modeled as a linear effect (P-value = 0.0008). There was no statistically significant evidence of nonlinearity in the association between NDI and TL (P-value = 0.1848).
