the standard
    practice in model fitting R functions with a <code>family</code> argument
    (e.g., <code><a href="https://rdrr.io/r/stats/glm.html" class="external-link">glm</a></code>) in which the default
    family is <code><a href="https://rdrr.io/r/stats/family.html" class="external-link">gaussian()</a></code>, in <code>mixed_model()</code> no default is provided. If the
    users wish to fit a mixed model for a Gaussian outcome, this could be done with
    function <code>lme()</code> from the <b>nl