. The argument is ignored for families that do not have a dispersion parameter. For an explanation of the dispersion parameter for each family, see <code><a href="sigma.glmmTMB.html">sigma</a></code>. The dispersion model uses a log link. In Gaussian mixed models, <code>dispformula=~0</code> fixes the residual variance to be 0 (actually a small non-zero value), forcing variance into the random effects. The precise value can be controlled via <code>control=glmmTMBControl(zero_dispval=...)</code>; the default