Research paper Table 1 Non-linear model fits for associations with estimated age at diagnosis Variable Motor examination score Striatal volume Speeded finger tapping rate (mean) Consistency in self-timed finger tapping Word list learning (HVLT) Odour identification n 438 261 408 410 425 424 Adj R2 0.15 0.23 0.14 0.20 0.09 0.10 p Value ,0.0001 ,0.0001 ,0.0001 ,0.0001 ,0.0001 ,0.0001 Non-linear df 3 1 3 3 1 1 Non-linear p value ,0.0001 ,0.001 ,0.0001 ,0.0001 ,0.01 ,0.0001 Adj R2 = partial adjusted variance accounted for by estimated years to diagnosis to Huntington’s disease (HD) after accounting for covariates (see statistical methods section) and degrees of freedom used for non-linear fit. HVLT, Hopkins Verbal Learning Test; Non-linear df, degrees of freedom for non-linear fit; non-linear p value, p value for non-linear element of estimated years to HD diagnosis fit. (variance explained) estimate. All models were adjusted for participant age, gender and, in the case of cognitive outcomes, years of education and estimated premorbid IQ (as reflected by Anart test scores). We made these adjustments by including the additional terms as conventional multiple regression covariates. As a secondary analysis, clinical motor score was also controlled in order to better interpret the results of psychomotor test analyses. We performed residual diagnostics on all models. We found no need to remove more than one outlier in any instance. Such outliers had small effects on details of the curve fit, but there was no occasion where they would have changed the statistical significance of the models. Spline model results are not readily interpreted in terms of traditional regression coefficients. Instead, we present graphs of the fitted curves, along with partial adjusted R square (‘‘variance explained’’) contributed by the expected time to diagnosis and the p values for both the total and non-linear contributions of expected time to diagnosis. Statistical signifi- cance attached to the non-linear contribution reflects the Figure 3 Relationship between estimated years to diagnosis of Huntington’s disease (HD) and various other measures. Solid line plots the predicted response; broken lines are 95% confidence limits for the estimated mean response. All relationships are adjusted to a female with mean levels of the other variables for which we adjusted our models (age = 41.2 years, education = 14 years, premorbid IQ). J Neurol Neurosurg Psychiatry 2008;79:874–880. doi:10.1136/jnnp.2007.128728 877