  Generalized linear models usually assume a common dispersion parameter. This
assumption is seldom true in practice, and may cause appreciable loss of type I
error control if standard parametric methods are used. We present an
alternative semi-parametric group invariance method based on sign flipping of
score contributions. Our method requires only the correct specification of the
mean model, but is robust against any misspecification of the variance. The
method is available in the R library flipscores.
