Dear All, I am attempting to perform an ANOVA with three factors: feature (3 levels), group (5 levels), and patient (246 levels), where patient is nested within group. I am using the following command: fit <- lm(intensity ~ feature + group + feature:group + group/patient, data = new) When using summary(fit), there are many parameter estimates of patient within group with NA values, and I receive this message: "Coefficients: (984 not defined because of singularities)". My understanding is that this is because each patient only occurs within a single level of group. But when I use the estimable function to estimate contrasts that involve certain patients, I get errors such as: Error in estimable.default(fit, cm) : Dimension of structure(c(1, 0, 1, 0, 0, 0, 0.5, 0, 0, 0, 0, 0, 0, 0, 0, 0, : 1x1235, not compatible with no of parameters in fit: 251 I'm wondering if the NA's are to be expected, and if so, how to perform contrasts involving levels of patient in the presence of the NA's. Any help is appreciated, Tim